138         DEMONSTRATIONS   IN    PHYSICAL   CHEMISTRY 

(4)  i. 60  milli-equivalents  of  Na2SO4.* 

(5)  5-°°  milli-equivalents  of  NaCl. 

(6)  50.00  milli-equivalents  of  NaCl.* 

(7)  5.00  milli-equivalents  of  MgCl2. 

(8)  50.00  milli-equivalents  of  MgCl2.* 

is  added  50  cc.  of  the  ferric  hydroxide  sol,  whereupon 
precipitation  is  observed  in  the  cases,  marked  by  an 
asterisk. 

169.  "Protective  Colloids." — The  use  of  emulsoids  in 
preventing   the   precipitation    of    dispersoids    is    demon- 
strated as   follows:1     Adding  first  200  cc.   of  a  N/5O 
sodium  chloride  solution  to  200  cc.  of  a  N/5O  silver  ni- 
trate  solution,    containing   5    cc.   of    strong   nitric   acid 
(specific  gravity  1.42),  a  white  flocculous  precipitate  im- 
mediately forms. 

The  experiment  is  then  repeated  with  equally  strong 
solutions  of  both  salts,  containing  I  per  cent,  of  gelatin 
dissolved.  The  mixture  becomes  opalescent,  and  the 
turbidity  increases  after  a  while,  without  forming  a  pre- 
cipitate. 

170.  The  deflocculation  of  suspensions  by  the  addition 
of  a  small  amount  of  acid  and  the  stabilizing  effect  of 
hydroxyl-ions  are  readily  demonstrated  as  follows: 

Ordinary  China  clay  is  stirred  up  in  water,  so  as 
to  form  a  suspension,  which  settles  out  rather  quickly, 
leaving  a  clear  liquid  above  and  a  sharply  defined  sedi- 
ment below.  If,  however,  a  little  alkali,  or  a  salt  with 
alkaline  reaction  is  added,  it  will  be  observed  that  the 

1  Noyes,  1.  c.  p.  91. 


COI^OIDS   AND   ADSORPTION  139 

settling  takes  place  much  more  slowly,  the  smallest  par- 
ticles not  settling  out  at  all,  or  if  so  only  very  gradually. 

171.  The  mobility  of  a  clay  suspension  containing  a 
little  acid  is  very  much  less  than  that  of  the  same  sus- 
pension with  a  trace  of  alkali  as  may  be  shown  by  allow- 
ing the  suspensions  (which  must  be  rather  concentrated) 
to  flow  down  an  inclined  glass  plate. 

172.  With  a  suspension  of  colophony  (rosin)  the  de- 
flocculation   by   one    drop   of    acid   is    a   very    striking 
phenomenon.      An   opaque  suspension   of   a  milky  ap- 
pearance is  obtained  by  dissolving  0.5  gram  rosin  in  10 
cc.  of  alcohol  and  pouring  the  solution  in  90  cc.  water. 
On  adding  one  drop  of  5N  hydrochloric  acid  an  immedi- 
ate deflocculation  takes  place.    A  small  amount  of  alkali 
dissolves  the  flocks  with  the  formation  of  a  soap. 

F.  Adsorption. 

Adsorption  includes  a  number  of  closely  related 
phenomena,  sometimes  distinguished  as  (i)  adsorption, 
(2)  absorption,  occlusion  or  solution  and  (3)  formation 
of  absorption  compounds.  A  sharp  demarcation  between 
these  groups  is  impossible.  In  some  cases,  e.  g.,  that  of 
palladium,  taking  up  hydrogen,  it  is  likely  that  all  three 
phenomena  occur.  In  order  to  avoid  these  cumbrous 
distinctions  some  authors  speak  of  "sorption."  The  fol- 
lowing mostly  well-known  experiments  on  sorption  or, 
— using  the  more  familiar  term  adsorption  as  a  general 
designation — on  adsorption  refer  to  the  condensation  of 
(a-)  gases,  (b)  liquids,  and  (c)  dissolved  substances  on 
different  solids. 


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LECTURE  DEMONSTRATIONS 

IN 

PHYSICAL  CHEMISTRY 


Published  by 

The  Chemical  Publishing  Company 


EASTON,  PA. 

Publishers  of  Scientific  Books 

Engineering  Chemistry  Portland  Cement 

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Household    Chemistry  Chemists'  Pocket  Manual 

Metallurgy,  Etc. 


Lecture  Demonstrations 


IN 


PHYSICAL  CHEMISTRY 


BY 


HENRY  S>VAN  KLOOSTER,  Ph.D. 

of  the  Department  of  Chemistry,  Rensselaer 
Polytechnic  Institute,  Troy,  N.  Y. 


EASTON,  PA. 

THE  CHEMICAL  PUBLISHING  CO. 
1919 

LONDON,    ENGLAND  TOKYO,    JAPAN 

WILLIAMS  &  NORQATE  MARUZEN  COMPANY,   LTD., 

14  HENRIETTA  STREET,  COVENT  QARDEN,  W.  C.  11-10    NIHONBA8HI  TORI-8ANCHOME 


COPYRIGHT,  1919,  BY  EDWARD  HART. 


CONTENTS. 


PAGE 

Preface v 

CHAPTER  I. 
General  Properties  of  Matter  in  the  Liquid  and  Solid  State ...       I 

CHAPTER  II. 
Diffusion 15 

CHAPTER  III. 
Osmosis 24 

CHAPTER  IV. 
Vapor  Pressure  and  Determination  of  Molecular  Weights 31 

CHAPTER  V. 
Chemical  Equilibrium  and  the  Law  of  Mass  Action 38 

CHAPTER  VI. 
Catalysis 55 

CHAPTER  VII. 
Electrochemistry  and  Ionic  Theory 67 

CHAPTER  VIII. 
Solubility  and  Its  Changes 108 

CHAPTER  IX. 
Colloids  and  Adsorption 114 

CHAPTER  X. 
Actino-chemistry  149 

CHAPTER  XI. 
Flame,  Combustion  and  Explosion 159 

CHAPTER  XII. 
Liquid  Air  Experiments 179 

Bibliography 188 

Index  of  Authors 190 

Index  of  Subjects 193 


"Quoniam  menti  humanae  nulla  corporum 
"vel  qualitatum  corporearum  est  innata 
"cognitio:  omnia,  quae  ad  corpora  perti- 
"nent,  observationibus,  vel  experimentis 
"addiscenda  sunt." 

—PETRUS  VAN  MUSSCHENBROEK, 

Introductio  ad  Philosophiam 
Naturalem,  p.  4  (1742). 


PREFACE. 

This  volume  of  lecture  demonstrations  has  been  prepared  with 
the  idea  that  it  would  be  of  service  to  have  a  set  of  experiments 
at  hand,  suitable  to  be  shown  in  the  lecture  for  the  illustration 
of  our  present  conceptions  on  physical  chemistry. 

Arrhenius,  in  the  introduction  to  his  "Theory  of  Solutions" 
states  "that  there  are  very  few  doctrines  in  exact  science,  where 
so  few  lecture  experiments  are  shown  as  in  physical  chemistry." 
This  is,  of  course,  partly  due  to  the  fact  that  quantitative  meas- 
urements are  needed  on  which  the  general  laws  must  be  based, 
while  lecture  experiments,  as  a  rule,  can  only  illustrate  the  prin- 
ciples involved  in  a  qualitative  way.  It  may  be  said,  however, 
that  quite  a  number  of  experiments  well  adapted  to  illustrate  the 
different  chapters  of  physical  chemistry  can  be  performed.  Some 
of  these  are  found  in  any  of  the  well-known  standard  works  of 
Heumann,  Arendt,  Newth  and  Benedikt,  but  little  or  no  attention 
is  paid  in  these  text-books  to  physical  chemistry  as  a  separate 
branch  of  teaching,  as  the  connecting  link  between  chemistry  and 
physics.  In  fact,  the  interesting  topics  of  physical  chemistry 
such  as  osmosis,  diffusion,  catalysis  are  treated  in  connection  with 
some  element  or  compound,  the  properties  of  wrhich  are  under 
discussion,  thereby  unconsciously  and  perhaps  unwillingly  intro- 
ducing the  idea,  that  these  phenomena  are  typical  or  especially 
characteristic  of  certain  elements  or  compounds.  To  take  a  few 
instances  out  of  many:  absorption  is  a  standing  property  of 
charcoal,  colloids  are  discussed  in  connection  with  silicon,  allo- 
tropy  is  taken  up  with  oxygen  and  ozon  a.  s.  o.  The  scope  of 
this  volume  is  diametrically  opposed  to  this  system  in  so  far  that 
relationships,  rather  than  distinctions  are  emphasized,  the  general 
character  of  the  different  topics  is  stressed  and  the  all-embracing 
grip  of  physical  or — as  it  is  frequently  called — general  chemistry 
underlined. 

It  is  interesting  to  note  as  can  be  seen  from  the  references, 
which  have  been  given  wherever  available,  that  many  experi- 
ments along  this  line  originate  from  the  great  masters,  which 


VI  PREFACE 

have  given  to  the  science  of  physical  chemistry  a  place  in  the 
front  ranks  of  exact  sciences.  The  very  fact,  that  chemists  like 
Faraday,  Graham,  Ostwald,  Fischer  and  others  have  spent  part 
of  their  time  in  devising  suitable  demonstrative  experiments  is 
sufficient  proof  for  the  usefulness  of  lecture  experiments,  wher- 
ever practicable,  even  in  the  case  of  such  a  "theoretical"  subject 
as  physical  chemistry.  However  important  the  theoretical  part 
may  be,  the  experimental  side  will  remain  our  first  and  our  final 
resort;  to  quote  the  words  of  an  early  Dutch  physicist,  cited  on 
a  preceding  page  in  the  original  version :  "Since  the  human 
mind  has  no  innate  knowledge  of  matter  or  its  properties,  every- 
thing pertaining  to  matter  must  be  learned  by  observation  and 
experiment." 

It  is  hoped  that  this  volume  will  be  useful  in  the  preparation 
of  lecture  experiments  and  stimulate  the  interest  of  the  students 
in  "practical"  physical  chemistry. 

Any  remarks  or  suggestions  as  to  changes  or  additions  will  be 
gladly  welcomed. 

The  author  takes  pleasure  in  stating  his  indebtedness  to  Prof. 
Bingham,  of  Lafayette  College,  for  the  help  received  in  correct- 
ing the  manuscript  and  giving  valuable  additions  (Nos.  I,  14,  170, 
171,  172  on  pp.  i,  12-14,  J38  and  139).  Acknowledgment  is  also 
expressed  to  Prof.  Hart  and  Dr.  Hunt  Wilson,  both  of  Lafayette 
College,  and  to  Dr.  van  Rossen,  of  Bryn  Mawr  College,  for  a 
number  of  suggestions.  In  the  reading  of  the  proof  sheets  the 
writer  was  assisted  by  Miss  M.  S.  Cline,  of  the  Moravian  College 
for  Women,  and  by  Mr.  Ch.  F.  Fryling  and  in  the  preparation  of 
the  cuts  by  Mr.  R.  ResnikofT,  to  whom  full  credit  for  their  pains- 
taking labor  is  hereby  given.  v.  K. 

WASHINGTON,  D.  C., 
August,  1918. 


CHAPTER  I. 


GENERAL  PROPERTIES  OF  MATTER  IN  THE 
LIQUID  AND  SOLD)  STATE. 

Fundamental  to  the  study  of  chemistry  and  physics  is 
the  differentiation  of  matter  into  the  solid,  liquid  and 
gaseous  states.  A  distinction  between  a  liquid  and  a  gas 
is  easily  made,  since  they  can  only  merge  into  each  other 
at  the  critical  point,  the  constants  of  which  (critical  tem- 
perature and  pressure)  are  readily  denned.  Solids  are 
usually  denned  as  having  a  definite  form  and  a  definite 
shape,  while  liquids  have  their  own  definite  volume,  but 
take  on  the  shape  of  the  vessel  in  which  they  are  con- 
tained. These  simple  definitions  do  not  hold,  however, 
in  the  case  of  very  viscous  or  plastic  substances  like 
glass,  pitch,  sealing  wax,  clay  and  similar  materials.  A 
sharp  demarcation  between  a  solid  and  a  liquid  is  pos- 
sible by  defining  a  solid  as  a  substance  which  requires  a 
definite  shearing  force  to  produce  a  permanent  deforma- 
tion. A  liquid  on  the  other  hand  is  permanently  de- 
formed by  any  shearing  force,  no  matter  how  small.1 
This  may  be  effectively  demonstrated  as  follows : 

1.  A  bar  of  pitch  is  made  up  I  centimeter  square  and 
10  centimeters  long.  A  similar  bar  is  made  of  modeling 
clay  and  both  laid  horizontally  on  two  supports,  9  centi- 
meters apart.  After  a  time,  which  depends  on  the  tem- 
perature, the  clay  bar  remains  perfectly  straight,  while 
the  pitch  bar  has  flowed,  showing  its  essentially  liquid 
condition. 

1  Bingham,  An  Investigation  of  the  I^aws  of  Plastic  Flow,  Bulletin  Bureau 
of  Standards,  No.  278,  p,  309,  (1916). 


2  DEMONSTRATIONS    IN    PHYSICAL    CHEMISTRY 

Starting  again  with  two  other  bars  of  exactly  the  same 
dimensions  a  load  of  100  grams  is  placed  upon  the  pitch 
bar  for  a  moment  only.  No  perceptible  sag  is  noted.  On 
placing  the  same  weight  upon  the  bar  of  plastic  clay,  it 
gives  way  completely.  The  clay  is,  therefore,  a  soft  (or 
plastic)  solid,  and  the  pitch  a  very  viscous  liquid. 

Among  the  properties  of  chemical  compounds  in  the 
liquid  and  solid  state,  which  are  most  suitably  illustrated 
by  lecture  demonstrations  may  be  mentioned  the  phase 
transitions  which  are  brought  about  by  a  change  of  tem- 
perature or  pressure.  Since  1884,  when  the  importance 
of  the  phase  rule  as  a  guiding  principle  for  the  rational 
classification  of  heterogeneous  equilibria  was  gradually 
recognized,  a  very  considerable  amount  of  work  on  phase 
transitions  in  general  has  been  done  by  Van't  Hoff, 
Bakhuis  Roozeboom,  Tammann,  Bancroft  and  their  co- 
workers.  It  is  safe  to  say  that  their  results  could  hardly 
ever  have  been  successfully  mastered  without  the  aid  of 
the  law  which  was  put  forward  by  Willard  Gibbs  in  1874. 

The  following  experiments  on  phase  transitions  deal 
with: 

A.  Polymorphic  transformations  of  compounds. 

B.  Dissociation  of  solids. 

C.  Undercooled  liquids. 

D.  Liquid  crystals. 

E.  Allotropy. 

F.  Passivity. 

The  chapter  is  concluded  with  a  demonstration  of  the 
relation  which  apparently  subsists  between  the  specific 


GENERAL   PROPERTIES   OF    MATTER  3 

heat  and  the  atomic  weight  of  elementary  solids  (Dulong 
and  Petit's  law). 

A.    Polymorphic  Transformations  of  Compounds. 

Although  it  has  been  known  for  a  long  time,  that  cer- 
tain compounds  exist  in  two  or  more  polymorphic  modi- 
fications, the  recognition  of  the  general  character  of 
polymorphism  dates  from  the  recent  investigations  by 
Tammann  and  others  on  the  polymorphism  of  a  great 
many  inorganic  compounds  (water  and  various  salts). 
The  greatly  improved  methods  for  the  measurement  of 
temperatures,  due  to  the  introduction  of  thermo-elements 
in  physico-chemical  work,  bring  us  daily  in  contact  with 
an  ever-increasing  number  of  polymorphic  compounds. 
The  transition  of  one  solid  phase  into  another  is  usually 
made  evident  by  the  heat  effect  at  the  transformation 
temperature ;  sometimes  also  by  a  marked  change  in  color 
or  a  noticeable  increase  or  decrease  in  volume. 

2.  The  change  in  color  is  easily  observed  by  inserting 
a  test-tube  with  5-10  grams  of  cuprous  mercuric  iodide  in 
a  beaker,  containing  water  of  about  80°.  The  color  of 
the  compound  changes  from  red  to  black.  The  color  is 
reversed  by  dipping  the  tube  in  water  of  50°,  or  by  allow- 
ing the  tube  to  cool  in  the  air.  In  preparing  this  double 
salt,1  mercuric  iodide  is  precipitated  from  a  solution  of 
6.8  grams  of  mercuric  chloride  in  100  cc.  of  water  by 
the  addition  of  50  cc.  of  a  solution  containing  8.3  grams 
of  potassium  iodide.  The  precipitate  is  washed  out  and 
dissolved  in  a  solution  of  8.3  grams  of  potassium  iodide 

1  cf.  H.  und  W.  Biltz,  Uebungsbeispiele  aus  der  unorg.  Experimental- 
Chemie,  Leipzig,  p.  27,  (1907). 


4  DEMONSTRATIONS   IN    PHYSICAL   CHEMISTRY 

in  50  cc.  of  water.  The  filtered  solution  is  mixed  with  a 
concentrated  solution  of  12  grams  of  copper  sulphate  in 
water  and  the  mixture  reduced  with  sulphur  dioxide. 
The  precipitate  is  thoroughly  washed,  dried  at  90-100° 
and  kept  in  a  closed  tube. 

In  lecture  courses  mercuric  iodide  is  usually  taken; 
this  substance,  however,  has  the  disadvantage,  that  the 
reverse  change  (on  cooling)  from  yellow  to  red  proceeds 
rather  slowly,  the  transition  temperature  (126°)  is  fre- 
quently overshot  by  more  than  100°,  and  it  requires  sev- 
eral hours,  sometimes  a  day  or  more  to  complete  the 
transformation.  The  reversible,  enantiotropic  character  of 
most  phase  transitions  is  therefore  more  clearly  demon- 
strated in  the  case  of  cuprous  mercuric  iodide  than  with 
the  latter  substance. 

3.  A  considerable  change  in  volume  at  the  transforma- 
tion from  one  modification  into  another  occurs  in  the 
case  of  potassium  tungstate.  This  salt  is  easily  prepared 
by  fusing  dry  potassium  carbonate  with  (previously 
ignited)  tungsten  trioxide.  It  is  exceedingly  hygroscopic 
and  must  be  kept  in  closed  tubes.  It  melts  at  921 0,x  and 
has  one  transition  point  at  388°,  which  temperature  is 
far  overshot  on  cooling,  before  the  transformation  starts 
with  increase  of  volume.  Four  to  five  grams  of  this  salt 
are  fused  on  a  square  piece  of  platinum  or  nickel  foil 
over  a  Bunsen  flame.  On  solidifying  it  will  be  seen — 
keeping  the  foil  inclined  towards  the  audience — that  the 
solid  crust  crumbles  after  a  while  and  drops  as  a  fine 

1  Van  Klooster,  Zeitschr.  f.  anorg.  Chem.,  85,  p.  49,  (1914)- 


GENERAL   PROPERTIES   OF    MATTER  5 

dust  from  the  foil,  owing  to  the  expansion  during  the 
transformation.1 

4.  Another  instance  is  potassium    bichromate.2      On 
fusing  about  10  grams  in  a  thin-walled  test-tube,  and  al- 
lowing the  molten  salt  to  cool,  it  solidifies  at  397°,  form- 
ing triclinic  crystals,  which  change  at  236°, 3 — with  hardly 
any  perceptible  heat  evolution — into  a  powder,  causing 
the  tube  to  crack  by  the  expansion. 

B.  Dissociation  of  Solids. 

5.  The  dissociation,   which  a  number  of   solid   com- 
pounds undergo  on  heating  is  easily  exemplified  in  the 
case  of  ammonium  chloride  or  ammonium  carbamate. 
With  the  former  substance  the  demonstration  is  conven- 
iently carried  out  by  placing  a  little  ammonium  chloride 
near  the  middle  of  a  hard  glass  tube   (about  40  centi- 
meters long,  inner  bore  I  centimeter),  held  in  a  slightly  in- 
clined position  by  a  clamp,  fastened  to  a  ring  stand.      A 
loose  plug  of  asbestos  wool  is  placed  a  little  above  the 
salt,  and  two  strips  of  moist  litmus  paper  inserted,  a  blue 
paper  at  the  lower  end  and  a  red  paper  at  the  upper  end. 
The  salt  is  gently  heated,  and  dissociates  into  a  mixture 
of  hydrogen  chloride  and   ammonia  gas.     The  latter,  be- 
ing the  lighter  gas  of  the  two,  diffuses  more  quickly  than 
the  hydrogen  chloride,  with  the  result  that  the  blue  paper 
is  reddened  by  the  excess  of  hydrogen  chloride  in  the 
lower  part  of  the  tube  and  the  red  paper  is  turned  blue 
by  the  ammonia,  which  diffuses  faster  than  the  hydrogen 

1  Hiittner  and  Tammann,  ibidem,  43,  p.   215,  (1905). 

2  Mitscherlich,  Pogg.  Annalen,  28,    p.  120,  (1833). 

3  Zemczuzny,  Zeitschr  f.  anorg.  Chem.,  57,  p.  267,  (1908). 


DEMONSTRATIONS    IN    PHYSICAL   CHEMISTRY 


chloride.  This  experiment  also  serves  as  a  demonstra- 
tion of  atmolysis.  The  very  simple  arrangement  de- 
scribed above  for  demonstrating  the  heat-dissociation  of 
ammonium  chloride,  is  due  to  Fenton.1  Other  types  of 
apparatus  for  the  same  purpose  have  been  devised  by 
Pebal2  and  Than.3 

C.  Undercooled  Liquids. 

6.  The  familiar  phenomenon  of  an  undercooled  (also 
called:  supercooled)  liquid  may  be  conveniently  demon- 
strated with  sodium  thiosulphate  (Na2S2O3.  5H2O). 
About  100  grams  of  the  salt  are 
heated  in  a  flat-bottomed  bulb  flask 
of  250  cc.  The  compound  melts  at 
48°  and  the  molten  salt  is  allowed 
to  cool  to  about  30°.  By  closing 
the  flask  with  a  loose  plug  of  cotton 
wool — thus  preventing  the  access 
of  minute  crystals  or  dust  particles, 
which  occasionally  act  as  "germs" 
in  breaking  up  the  metastable  con- 
dition, the  supercooled  liquid  may 
be  kept  for  an  indefinite  time. 
Crystallization  can  only  be  started 
by  a  crystal  of  the  salt  (which  may 
be  almost  invisible).  By  introduc- 
ing a  glass  rod,  covered  at  its  lower  end  with  a  thin  crust 
of  the  solid  salt,  without  any  adhering  loose  powder, 

1  cf.  Mellor,  Modern  Inorganic  Chemistry  p.  542,  (1916). 

2  lyiebigs  Annalen  123,  p.  199,  (1862). 
*  Ibidem,  131,  p.  129,  (1864). 


Fig.   i. 


GENERAL  PROPERTIES  OF   MATTER  7 

into  the  undercooled  liquid,  crystallization  starts  from 
the  end  of  the  glass  rod  (with  simultaneous  evolu- 
tion of  heat)  and  after  a  few  seconds  the  rod  is  lifted  out 
of  the  liquid,  covered  with  a  conglomerate  of  crystals; 
(Fig.  i)  at  the  same  time,  however,  no  further  solidifica- 
tion is  observed  in  the  liquid,  due  to  the  fact,  that  the 
solid  phase  has  been  completely  removed.1 

7.  A  case,  analogous  to  the  crystallization  of  an  un- 
dercooled liquid  is  that  of  the  devitrification  of  a  (silicate) 
glass,  as  can  be  shown  with  sodium  metasilicate 
(Na,SiO3).  This  salt  melts  at  io88°,2  and  solidifies, 
when  slowly  cooled,  at  temperatures,  varying  from  1080°- 
1000°.  The  salt  is  easily  prepared  by  mixing  sodium 
carbonate  and  silica  (quartz)  in  equivalent  quantities, 
heating  the  mixture  for  1-2  hours  at  a  temperature  of 
6oo°-8oo°,  thereby  effecting  a  partial  combination.  The 
sintered  mass  is  pulverized  and  the  above  process  re- 
peated two  or  three  times,  in  order  to  insure  perfect 
homogeneity.  Finally  the  powder  is  fused  and  on  slowly 
cooling  changes  into  a  conglomerate  of  opaque  crystals. 
Ten  grams  of  the  salt  are  heated  in  a  small  platinum 
crucible  and  rapidly  cooled  by  means  of  a  stream  of  cold 
air,3  whereupon  a  perfectly  clear  and  transparent  glass  is 
formed.  This  glass  is  then  slowly  heated,  either  in  the 
crucible  or  on  a  piece  of  platinum  or  nickel  foil  over  a 
Bunsen  flame.  At  a  temperature  where  the  glass  just 
begins  to  soften  (about  550°)  the  devitrification  (crystal- 

1  Ostwald,  Grundlinien  der  anorg.  Chemie,  3e  Aufl,  p.  537,  (1912). 

2  Jaeger,  Journ.  of  the  Wash.  Ac.  of  Sc.,  1,  p.  53,  (1911). 
'"  Guertler,  Zeitschr.  f.  anorg.  Chem.,  40,  p.  268,  (1904). 

2 


8  DEMONSTRATIONS    IN    PHYSICAL   CHEMISTRY 

lization)  suddenly  starts,  often  accompanied  by  a  strong 
glowing,  indicating  an  enormous  increase  of  temperature. 

D.  Liquid  Crystals. 

8.  As  an  example  of  a  group  of  organic  compounds, 
which  are  characterized  by  two  melting  points,  the  case 
of  para-azoxyanisol  may  be  quoted.     This  substance,  dis- 
covered by  Gattermann,  melts  at  n  6°  to  a  turbid  bright 
yellow  liquid,  which  on  further  heating,  suddenly  clears  at 
135°.  The  phenomenon  is  suitably  projected  on  the  screen 
by  heating  the  substance  in  a  small  glass  trough  with 
parallel   walls   of   rectangular   cross   section.     The   first 
melting  point  (116°)  represents  the  conversion  of  a  crys- 
talline   solid    into    (anisotropic)    liquid    crystals,    which 
change  at  135°  into  an  (isotropic)  liquid. 

E.   Allotropy. 

The  recent  work  of  Cohen  and  his  co-workers  on  this 
topic  have  clearly  brought  out  the  frequent  occurrence  of 
polymorphism  among  elements,  especially  heavy  metals. 
Since  in  most  cases  the  change  from  one  solid  phase  into 
another  at  the  transition  point  is  accompanied  by  an  ap- 
preciable change  in  volume  the  method  chiefly  employed 
is  that,  which  makes  use  of  a  dilatometer. 

9.  The  following  lecture  experiment1  gives  a  good  idea 
of  the  enormous  decrease  in  volume,  resulting  from  the 
transformation  of  grey  tin  into  white.     At  the  tempera- 
ture of  transformation  (18°)  the  specific  gravities,  as  de- 
termined by  Cohen,2  are  5.79  and  7.28  respectively.    The 

1  Cohen,  Transactions  of  the  Faraday  Soc.,  7,  p.  6,  (1911). 

2  Zeitschr.  f.  phys.  Chem.,  30,  p.  601,  (1899). 


GENERAL   PROPERTIES   OF    MATTER  9 

dilatometric  apparatus  (Fig.  2),  consists  of  a  glass  cylin- 
der (A),  filled  with  60-70  grams  of  grey  tin,  and  a  con- 
necting U-shaped  tube,  containing  mercury.  The  space 


Fig.  2. 

between  mercury  and  tin  is  filled,  as  far  as  the  stopcock 
K,  with  distilled  water.  On  the  mercury  in  the  open 
limb  of  the  U-tube  floats  a  small  cylindrical  weight,  con- 
nected by  means  of  a  thin  thread  with  the  disk  S,  which 
turns  around  an  axis,  kept  in  its  place  by  the  beam  H.  A 
pointer  fastened  to  the  disk  and  moving  along  a  graduated 
scale,  follows  the  displacements  of  the  mercury  in  the 
U-tube.  The  zero-position  is  reached  by  opening  the 
stopcock  and  pouring  water  in  the  apparatus,  until  the 


IO  DEMONSTRATIONS   IN    PHYSICAL   CHEMISTRY 

pointer  is  adjusted.  The  stopcock  is  then  closed  and  the 
cylinder  A  warmed  up  with  water  of  about  8o.°  The 
mercury  sinks  in  the  open  limb  and  a  sudden  upward 
move  of  the  pointer  over  three  or  more  scale  divisions  is 
observed. 

It  has  been  found,  that  the  reverse  change,  from  the 
white  modification,  in  which  tin  is  usually  known,  into 
the  grey  form  goes  fastest  at  a  temperature  of  — 45°, 
and  also,  that  the  transformation  is  accelerated  in 
the  presence  of  pink  salt  solution.  In  the  absence  of 
the  grey  modification  white  tin  can  be  kept  below  18° 
several  months,  or  even  years,  without  the  slightest  indi- 
cation of  any  transformation.  If,  however,  the  white  tin 
is  "infected"  with  a  trace  of  grey  tin,  the  transformation 
goes  on,  until  the  "tin  pest"  has  entirely  affected  the 
white  modification. 

10.  The   phenomenon   of   dynamical  allotropy   in   the 
liquid  state  is  shown  by  sulphur  and  was  thoroughly  in- 
vestigated, first  by  A.  Smith  and  his  pupils,  and  after- 
wards by  Kruyt  and  his  co-workers.     The  peculiar  be- 
havior of  molten  sulphur  in  the  neighborhood  of   160° 
and  the  formation  of  plastic  and  amorphic  sulphur  are 
usually  demonstrated  in  first  courses  on  inorganic  chem- 
istry and  need  no  special  description  at  this  place.       It 
may  be  remarked,   that   from  a  colloid-chemical   stand- 
point this  behavior  is  interesting,  when  sulphur  is  con- 
sidered,— as  W°  Ostwald  proposes1 — as  an  "allo-colloid." 

F.  Passivity. 

11.  The  change  in  condition,  which  some  heavy  metals, 

1  Ostwald-Fiseher,  Handbook  of  Colloid  Chemistry,  p.  104,  (1915). 


PROPERTIES   OF    MATTER  II 


especially  iron  and  chromium  undergo,  when  inserted  in 
strong  nitric  acid  (specific  gravity  1.50),  usually 
called  "passivity,"1  may  be  demonstrated  in  the  following 
manner.  A  square  piece  of  thin  sheet  iron,  well  cleaned, 
is  attached  to  a  platinum  wire,  and  lowered  in  a  beaker 
containing  dilute  nitric  acid,  in  which  the  iron  is  imme- 
diately attacked.  It  is  then  transferred  to  another 
beaker  with  concentrated  nitric  acid  (specific  gravity 
1.50)  ;  nothing  happens.  Having  removed  the  adhering 
acid  by  inserting  the  iron  in  a  beaker  with  distilled  water, 
the  now  passive  iron  is  brought  in  a  fourth  beaker  con- 
taining a  dilute  solution  of  copper  sulphate.  No  film  of 
copper  is  formed  on  the  iron,  which  remains  grey  as  be- 
fore. Care  has  to  be  taken,  that  the  iron  is  not  touched 
in  some  way  or  other,  because  hammering,  bending  or 
scratching  immediately  restores  the  active  state  as  will 
be  seen  by  the  formation  of  a  thin  copper  coating. 

12.  The  iron  can  also  be  brought  in  the  passive  state  by 
dissolving  the  metal  electrolytically,  using  the  iron  as  an 
anode  in  electrolyzing  an  aqueous  solution  of  sulphuric 
(or  nitric  acid),  or  a  solution  of  a  nitrate  or  sulphate. 

Passivity  was  discovered  by  Keir2  and  studied  more  in 
detail  by  Faraday  (1836)  and  simultaneously  by  Schon- 
bein.  A  good  explanation  for  this  peculiar  phenomenon 
is  still  lacking.  Some  authors3  ascribe  the  activity  to  the 
presence  of  hydrogen  ions  at  the  surface  of  the  iron  ;  by 
dipping  the  metal  in  concentrated  nitric  acid  the  hydrogen 
is  oxidized  and  the  metal  becomes  passive.  Activity  is 

1  Schonbein,  Pogg.  Ann.  37,  pp.  390,  590,  (1836). 

2  Phil.  Transact.,  80,  p.  374,  (1790). 

3  cf.  Rathert,  Zeitschr.  f.  phys.  Chera.,  86,  p.  567,  (1914). 


12  DEMONSTRATIONS   IN    PHYSICAL   CHEMISTRY 

restored  by  heating  in  hydrogen  gas  or  inserting  the  metal 
as  a  cathode  in  a  ferrous  sulphate  solution.  Another 
explanation,  first  advanced  by  Faraday,  traces  the  cause 
of  passivity  to  the  formation  of  a  protecting  skin  of  oxide. 
Quite  recently  Smits1  has  given  an  entirely  new  explana- 
tion, based  on  the  assumption  of  different  kinds  of  mole- 
cules or  ions  in  the  metal,  which  are  in  mobile  equilibrium. 
Passivity,  according  to  Smits,  would  be  nothing  but  a  dis- 
turbance of  this  internal  equilibrium. 

13.  The    following    experiment,    taken    from    Smits' 

^rx^ paper,    shows   that    passivity    can    be 

"  overcome  by  bringing  the  iron  in  con- 
tact with  solutions  of  chlorides,  bro- 
mides, or  iodides,  a  fact,  which  cannot 
well  be  reconciled  with  the  oxide 
theory.  A  piece  of  sheet  iron,  pro- 
vided with  an  elbow  (Fig.  3),  is  first 
inserted  in  strong  nitric  acid,  and 


Fig.  a.  then  in  a  concentrated  solution  of  cop- 

per sulphate.  No  copper  is  deposited,  but  on  bringing  the 
elbow-appendix  in  contact  with  a  solution  of  potassium 
chloride,  bromide,  or  iodide,  activity  is  restored  at  once. 
A  solution  of  mercuric  chloride  has  no  effect,  hence  the 
activating  action  is  exerted  by  the  Cl/  Br'  and  I'-ions 
respectively. 

14.  The  law,  discovered  by  Dulong  and  Petit  in  1819, 
stating  that  the  heat  capacity  of  atoms  is  approximately 
the  same  for  all  solid  elements,  is  very  striking  as  is 
readily  seen  from  the  following  table  which  contains  a 

i  Chem.  Weekblad  12,  p.  676.  (1915). 


PROPERTIES   OF    MATTER 


number  of  elements  (metals)  selected  at  random  from  a 
list  of  more  than  fifty  elements  arranged  in  the  order  of 
increasing  atomic  weight: 


Element 

Atomic 
weight 

Specific 
heat 

Specific 
gravity 

Atomic 
volume 

Atomic 
heat 

Aluminium  .... 

27.1 
65.4 

0.2T7 
O.OQ4 

2.7 

7.1 

10.4 
0.2 

5-9 
6.1 

Tjn     

118  7 

o  0^*1 

7  ^ 

16  •; 

6  s 

Lead  

2O7  2 

O  O^I 

1  1  7 

18  2 

64 

The  importance  of  this  law  is  frequently  not  realized 
to  its  full  extent,  especially  in  elementary  courses  of 
inorganic  chemistry  because  of  the  lack  of  a  suitable  lec- 
ture demonstration.  This  is,  however,  a  very  simple  mat- 
ter, since  it  is  merely  necessary  to  take  amounts  of  two 
elements  in  ratio  of  their  atomic  weights,  heat  them  to 
1 00°  and  then  plunge  them  in  equal  volumes  of  water  at 
room  temperature.  The  rise  in  temperature  is  approxi- 
mately the  same  as  can  be  readily  seen  at  a  distance  by 
using  two  large  air  thermometers  of  equal  size.  The 
apparatus  used  by  Prof.  Bingham  in  his  lectures1  consists 
of  a  lead  weight  of  4144  grams  (20  gram  atoms)  and  a 
zinc  weight  of  1308  grams  (20  gram  atoms)  of  the  same 
cross  section  (10  centimeters  square)  as  shown  in  Fig.  4. 
From  the  table  given  above,  it  is  seen  that  the  atomic 
volume  of  lead  (18.2)  is  twice  that  of  zinc  so  that  the 
volume  of  the  lead  weight  as  judged  from  its  height  must 
be  double  that  of  the  zinc  weight.  Both  weights,  provided 
with  brass  handles  for  ease  of  manipulation  are  heated 
in  a  pail  (or  dish)  containing  boiling  water,  simultane- 

1  Obtainable  from  Eimer  and  Amend,  New  York. 


DEMONSTRATIONS    IN    PHYSICAL   CHEMISTRY 


ously  removed  and  plunged  into  two  glass  jars  of  equal 
size  (diameter  16  centimeters,  height  9  centimeters) 
filled  with  500  cc.  of  water  at  room  temperature.  In 
the  center  of  the  jars  are  placed  two  air  thermometers 


Fig.  4. 

filled  with  a  colored  liquid  and  carefully  adjusted,  so  that 
both  show  the  same  rise  of  liquid  in  the  stem  for  equal 
increments  of  temperature.  The  initial  position  of  the 
liquid  in  the  stem  is  marked  by  means  of  a  clip.  The 
rise  of  liquid  will  be  found  to  be  several  centimeters 
(dependent  upon  the  bore  of  the  thermometer  stem)  but 
the  same  for  both  thermometers. 

If  desired,  the  experiment  may  be  repeated  using  equal 
-weights  of  lead  and  zinc  in  which  case  the  rise  of  tem- 
perature will  be  more  than  three  times  greater  for  the 
zinc  than  for  the  lead. 


CHAPTER  II. 


DIFFUSION. 
I.   Diffusion  in  Gases. 

15.  The  process  of  diffusion  of  gases  has  been  the  sub- 
ject of  exhaustive  researches  by  Graham  (1832),  to 
whom  we  owe  the  laws  governing  gaseous  diffusion  and 
the  related  phenomena  of  effusion  and  transfusion.  As 
Graham  has  shown,  the  relative  speeds  of  diffusion  of 
gases  are  inversely  proportional  to  the  square  roots  of 
their  relative  densities.  That  hydrogen,  being  the  light- 
est of  all  known  gases,  diffuses  faster  than  air  through 
the  walls  of  a  thin  porous  membrane,  while  air  itself, 


Fig.  5. 

being  lighter  than  carbon  dioxide  travels  faster  through 
the    membrane    than    does    carbon    dioxide,    is    readily 


l6  DEMONSTRATIONS   IN    PHYSICAL   CHEMISTRY 

shown  by  the  use  of  unglazed,  porous,  porcelain  cylin- 
ders, connected  with  a  long  narrow  glass  stem,  as  were 
first  recommended  for  this  purpose  by  Wohler.1  The 
whole  arrangement  may  be  seen  from  the  figure  (Fig.  5). 
Both  cylinders  contain  air,  under  atmospheric  pressure 
as  indicated  by  the  open  manometers,  with  which  the 
stems  are  connected.  An  inverted  beaker  filled  with 
hydrogen  is  brought  over  one  pot  and  another  beaker 
filled  with  carbon  dioxide  over  the  second  cylinder.  The 
different  speed  of  diffusion  instantly  causes,  in  one  case, 
a  (temporary)  excess  of  pressure,  and  in  the  other  a 
reduction  of  pressure,  until,  after  a  few  minutes, 
equilibrium  is  re-established.  On  removing  the  beakers 
the  reverse  takes  place. 

16.  The  different  speed  of  diffusion  can  also  be  demon- 
strated in  an  elegant  manner  by  the  use  of  small  glass 
bulbs  (of  about  1.5-2  centimeters  in  diameter)  filled  with 
liquid  bromine,  as  used  by  Biltz.2  These  bulbs  are 
made  by  drawing  out  a  glass  tube  into  capillary  ends  and 
blowing  the  intermediate  piece  of  tubing  into  the  re- 
quired shape.  Two  of  these  bromine  bulbs  are  placed 
in  two  glass  cylinders  (height  27  centimeters,  width  6.5 
centimeters)  closed  at  both  ends  by  well-fitting  glass 
plates  coated  with  a  little  grease,  so  as  to  insure  gas- 
tight  connections.  The  upper  glass  plates  are  perforated 
and  closed  by  rubber  or  cork  stoppers.  One  of  the  stop- 
pers has  one  hole,  which  allows  the  passage  of  a  long 
glass  rod,  bent  at  right  angles  at  its  end  in  the  form  of  a 
circle,  in  order  to  crush  the  bromine  bulb  at  the  proper 

1  Ber.  d.  chem.  Ges.,  4,  p.  10,  (1871). 

2  Zeitschr.  f.  phys.  Chem.,  9,  p.  152,  (1892). 


DIFFUSION 


moment.  The  other  stopper  has  two  holes,  through 
which  passes  a  glass  tube  (inner  bore  0.4  centimeter) 
similar  in  shape  to  the  glass  rod  in  the  first  mentioned 
cylinder  and  serving  for  a  like  purpose,  and  another  L,- 
shaped  tube,  provided  with  a  piece  of  rubber  tubing  and 
a  pinchcock  for  the  introduction  of  hydrogen  gas  (Fig. 
6).  The  dry  gas  is  passed  through  in  a  rapid  stream, 


Fig.  6. 

expelling  at  the  same  time  the  air  through  the  "crushing" 
tube.  After  2  minutes  the  cylinder  is  filled  and  both 
tubes  closed  by  the  pinchcock  and  a  cork  stopper  re- 


l8  DEMONSTRATIONS    IN    PHYSICAL   CHEMISTRY 

spectively.  The  two  bromine  bulbs  are  crushed  simul- 
taneously and  the  difference  in  behavior  of  the  air- 
bromine  and  hydrogen-bromine  mixture  becomes  visible 
in  the  course  of  3-5  minutes.  Using  white  screens  to 
make  the  colors  visible  at  a  distance,  it  will  be  seen  that 
in  the  hydrogen  cylinder  the  bromine  fills  the  space  half 
way  up,  while  in  the  air  cylinder  the  bromine  has  moved 
only  one-fourth  upward. 

17.  On  the  different  speed  of  diffusion  through  a 
porous  septum  is  based  a  method  first  applied  by  Gra- 
ham, called  atmolysis  to  separate  one  gas  from  another. 
Ostwald1  has  given  the  following  arrangement  to  show 
the  separation  of  detonating  gas  into  hydrogen  and  oxy- 
gen by  this  method. 

The  gas  is  generated  in  a  wide  mouth  bottle  (Fig.  7), 


Fig.  7. 

filled  with  a  rather  strong  solution  of  caustic  soda,  which 
is  electrolyzed  by  the  current  from  two  storage  cells. 
Two  cylindrical  iron  or  nickel  sheets  are  used  as  elec- 
trodes. The  gas  is  dried  by  a  U-tube  filled  with  granu- 
lated calcium  chloride  and  enters  first  the  left  (glass) 

1  Ostwald-McGowan,  The  Scientific  Foundations  of  Anal.  Chem.,  srded., 
p.  232,  (1908). 


DIFFUSION  19 

arm  of  a  branched  tube,  the  one  stopcock  being  turned 
on  and  the  other  turned  off;  the  gas  collected  in  a  test 
tube  over  water  explodes  with  a  lighted  match.  When 
however  the  gas  is  made  to  pass  through  the  right  tube 
of  unglazed  porcelain,  it  will  be  seen  that,  under  proper 
conditions  the  hydrogen  diffuses  out  almost  completely 
with  the  result  that  the  collected  gas  rekindles  a  glowing 
splint,  thereby  showing  that  it  is  oxygen  that  is  left. 


Fig.  8 


18.  The  same  effect  is  obtained  by  passing  the  elec- 
trolytic gas  through  two  crossed  "churchwarden"  clay 
pipes,  connected  by  a  piece  of  thick  walled  rubber  tubing 
(Fig.  8).  Both  in  this  and  in  the  preceding  experiment 


20 


DEMONSTRATIONS   IN    PHYSICAL   CHEMISTRY 


the  proper  rate  at  which  the  gas  mixture  travels  has  to 
be  found  out  beforehand.  If  the  gas  stream  is  too  rapid, 
the  hydrogen  has  no  time  to  diffuse  out ;  on  the  other 
hand,  if  the  rate  is  too  slow,  air  will  diffuse  into  the  tube 
so  that  a  glowing  splint  will  not  burst  into  flame. 

19.  That  the  law  of  diffusion  also  holds  good  for  ef- 
fusion, i.  e.,  the  passage  of  a  gas 
through  a  fine  orifice,  was  also 
found  by  Graham  (1832)  and 
may  be  shown  for  hydrogen  and 
oxygen  with  an  apparatus  de- 
vised by  Freer1  (Fig.  9),  con- 
sisting of  a  U-tube,  connected  on 
one  side  with  a  two-hole  stop- 
cock (A)  and  on  the  other  side 
with  a  barometer  tube.  The  left 
limb  of  A  (a)  contains  a  piece  of 
glass  rod  and  is  drawn  out  into 
an  extremely  narrow  tip,  while 
the  right  outlet  tube  (b)  is  left 
unchanged.  After  the  bend  of 
the  U-tube  has  been  covered 
with  mercury,  so  as  to  separate 
the  air  space  on  both  sides,  dry 
hydrogen  gas  is  passed  through 
the  tube,  escaping  through  b  (a 
being  closed).  After  a  few  minutes  A  is  turned  off,  and 
mercury  poured  in  the  long  limb  of  the  U-tube  up  to  a 
certain  height,  the  hydrogen  in  the  short  limb  occupying 

1  Zeitschr.  f.  phys.  Chem.,  9,  p.  669,  (1892). 


Fig.  9. 


DIFFUSION  21 

a  volume  of  about  80  cc.,  marked  off  by  a  strip  of  paper. 
The  gas  is  then  allowed  to  escape  through  the  tip  a,  a 
metronome  being  used  to  note  the  time  necessary  to  drive 
the  gas  out  to  a  mark  just  below  the  stopcock.  This 
ought  to  require  about  7  seconds.  The  experiment  is  then 
repeated  replacing  the  hydrogen  by  oxygen.  If  proper 
care  is  taken  in  filling  the  tube  with  an  equal  volume  of 
pure  oxygen,  the  time  of  effusion  will  be  four  times  as 
long  as  before. 

II.  Diffusion  in  Liquids. 

20.  Diffusion  in  liquid  state,  (and  taking  as  a  typical 
instance  that  of  salt  solutions  in  water),  first  carefully 
studied  by  Graham  (1850-51)  requires  such  a  consider- 
able time  to  show  a  visible  result  that  the  effect  of  diffu- 
sion can  only  be  seen  after  half  a  day 
or  longer.  The  experiment  may  be 
carried  out, — following  Graham's  di- 
rections— by  filling  a  bottle  with  a 
concentrated  solution  of  the  salt, 
(copper  nitrate  or  chloride)  and  plac- 
ing this  bottle  in  a  cylindrical  vessel 
which  is  then  filled  to  the  top  with  dis- 
tilled water  (Fig.  10). 


A  sharp  boundary  surface  between  Fis-  10- 

the  water  and  the  solution  may  also  be  obtained  by  con- 
necting the  cylinder  containing  the  water  through  a  bent 
capillary  glass  tubing  with  a  separatory  funnel  (Fig.  n), 
and  allowing  the  heavy  salt  solution  to  slowly  push  the 
aqueous  layer  upward  without  any  perceptible  mixing. 


22 


DEMONSTRATIONS    IN    PHYSICAL   CHEMISTRY 


Still  another  scheme  is  to  cover  the  salt  solution,  filling 

a  cylindrical  jar  halfway, 
with  a  thin  cork  disk  and  to 
allow  the  water  to  drop 
slowly  on  the  disk.  The 
original  sharp  demarcation 
line  between  the  two  layers 
disappears  as  the  diffusion 
progresses. 

III.  Diffusion  in  Solids. 

Diffusion  of  solids  into 
each  other,  requires  months 
and  years  to  show  a  notice- 
able result,  as  has  been 
demonstrated  by  the  work 
of  Roberts-Austen1  on  the 
diffusion  of  gold  in  lead  at 
20°,  100°  and  250°.  An 

experiment,  that  takes  a  few  weeks  and  illustrates  to  a 
certain  extent  the  diffusion  in  solids,  is  the  following: 

21.  A  5  per  cent,  solution  of  gelatine  in  water  (150  cc.) 
is  made  and  divided  in  three  equal  volumes.  One  part  is 
left  uncolored,  the  other  two  portions  are  dyed  with 
congo-red  and  methyl  violet,  or  any  other  organic  dyes. 
The  solutions  are  poured  in  three  crystallizing  dishes 
of  10  centimeter  diameter,  and  when  coagulated,  taken 
out  in  the  form  of  thick  plates,  which  are  placed  in  a 

1  Transactions  of  the  Royal  Soc.,  187,  p.  383,  (1896). 


Fig.  11. 


DIFFUSION  23 

large  glass  jar,  one  on  top  of  the  other,  the  uncolored 
plate  being  put  in  the  middle.  The  jar  is  covered  by  a 
cork  stopper  and  set  aside  in  a  suitable  place,  where  the 
result  of  the  diffusion  of  the  colors  can  be  observed  at 
any  time. 


CHAPTER  III. 


OSMOSIS. 

I.    Osmotic  Experiments  with  Gases. 
The  property  of  palladium,  especially  when  heated,  of 
dissolving  hydrogen  readily,  but  not  nitrogen  has  been 
used  by  Ramsay1  to  carry  out  osmotic  experiments  with 
a  nitrogen-hydrogen  mixture.     Since  it  is  necessary  to 
work  at  high  temperatures,  in  order  to  obtain  satisfactory 
results,  it  is  more  convenient  to  carry  out  a  similar  ex- 
periment at  the  ordinary  tempera- 
ture with  air  and  ammonia,  replac- 
ing the  palladium  by  animal  mem- 
brane     moistened      with      water. 
Ammonia   is   extremely   soluble   in 
water,  while  hydrogen,  oxygen  and 
nitrogen   are  difficultly   soluble   in 
this    solvent.      The    thin    film    of 
water  on  the  membrane  acts  in  this 
way  as  a  semipermeable  membrane. 
22.  A  thistle  tube  covered  with 
the  moist  membrane,  is  bent  in  the 
form   of   a    U,    and   contains   air, 
under  atmospheric  pressure  as  in- 
dicated by  the  height  of  some  col- 
ored oil  in  both  limbs  of  the  U-bend 
(Fig.   12.). 2     If  now  a  beaker  is 
inverted  over  the  head  of  the  thistle 


Fig.  12. 


i  Phil.  Mag.,  38,  p.  206,  (1894). 

»  Stieglitz,  Elements  of  Qualitative  Chemical  Analysis,  Vol.  I,  New  York 
(1916),  p.  22,  also:  Alex.  Smith,  Introduction  to  Inorg.  Chemistry,  3d  ed.,  New 
York,  p.  329,  (1917). 


OSMOSIS  25 

tube  and  hydrogen  admitted,  no  increase  of  pressure 
inside  the  thistle  tube  is  observed.  On  substituting  an 
atmosphere  of  ammonia  for  the  hydrogen,  the  gas  dis- 
solves quickly  in  the  water  on  the  membrane  until  satu- 
ration, and  then  enters  the  inside  of  the  tube  producing 
an  increase  in  pressure.  A  piece  of  red  litmus  paper 
changes  color  at  the  same  time. 

II    Osmotic  Experiments  with  Liquids. 

23.  A  very  simple  osmotic  experiment,  which  forms  a 
modification  of  the  original  experiment,  performed  by  the 
discoverer  of  osmosis,  the  abbe  Nollet1  was  described  by 
Lupke2  as  follows : 

A  100  cc.  glass  jar  is  filled  with  a  nearly  saturated 
solution  of  cane  sugar  and  closed  with  bladder.  On  sub- 
merging the  jar  in  water  the  volume  increases  consider- 
ably in  the  course  of  2  or  3  hours  and  the  membrane 
swells  up  to  such  an  extent,  that  on  piercing  the  latter 
with  a  thin  needle  a  stream  of  liquid,  about  20  centimeters 
high,  is  thrown  up. 

24.  The  realization  of  practically  semipermeable  mem- 
branes rests  on  the  discovery  by  M.  Traube  of  the  copper 
ferrocyanide  precipitation  membrane,  the  formation  of 
which  may  be  shown  in  a  way  suitable  for  projection,  in 
a  small  trough  with  parallel  walls  (Fig.  13),  filled  with 
a  half-saturated  solution  of  copper  sulphate.3     From  a 
pipette,  containing  a  nearly  saturated  solution  of  potas- 

1  M6moires  de  1'Ac.  Royale  des  Sc.,  p.  57,  (1748). 

2  Grundziige  der  Blectrochemie  50  Aufl.,  p.  91,  (1907). 

'c.f.  Nernst,  Theoretische  Chemie,  6e  Aufl.,  p.  133  (1909).  Thiel,  Zeitschr. 
f.  Electrochemie,  12,  p.  229,  (1906). 


26  DEMONSTRATIONS   IN    PHYSICAL   CHEMISTRY 

sium  ferrocyanide,  one  drop  is  allowed  to  fall  on  the 
copper  sulphate  solution.  An  exceed- 
ingly tenuous  membrane  of  the  brown 
copper  ferrocyanide  is  formed,  through 
which  water  passes  into  the  solution,  en- 
closed by  the  precipitate.  The  result  is, 
that  the  solution,  surrounding  the  drop 
becomes  more  concentrated  and  sinks  in 


Fig.  is.  thread-like  streaks  to  the  bottom.  These 
streaks  are  easily  seen,  owing  to  the  different  refractive 
indices  of  solutions  of  different  densities.  Tammann1 
has  used  this  method  of  "streaks"  in  an  ingenious  way 
to  detect  isosmotic  (isotonic)  solutions. 

25.  Precipitation    membranes,    like    the    above    men- 
tioned, are  obtained  in  a  similar  manner,  by  pouring  a 
moderately  concentrated  sodium  silicate  solution  (specific 
gravity  I,  i)  into  a  number  of  lecture  jars  containing  a 
few  crystals  of  copper-,  iron-,  manganese-,  nickel-  and 
cobalt  salts  respectively.     After  standing  over  night  in  a 
quiet  place,  peculiar,  coralline  shoots  (so-called  "chemi- 
cal gardens")  are  formed,  of  different  shape  and  color, 
characteristic  of  the  salts  used.     The  mode  of  formation 
is  the  same  as  in  the  case  of  the  copper  ferrocyanide. 

26.  The  precipitation  of  efficient  copper  ferrocyanide 
membranes  in  the  pores  of  unglazed  porcelain  cells  (after 
Pfeffer)    is    connected    with    considerable    experimental 
difficulties,  as  was  clearly  brought  out  by  Morse  and  his 
collaborators.     It  is,  therefore,  preferable  to  use  for  lec- 
ture experiments,  demonstrating  osmotic  pressure,  parch- 
ment thimbles  (as  may  be  obtained  from  Schleicher  and 

1  Wiedemaiis  Annalen,  34,  p.  299,  (i-sS). 


OSMOSIS 


Schiill,  in  the  dimensions  of  100  by  16  millimeters,  No. 
579),  tightly  fastened  to  a  long  narrow  tube1  (Fig.  14). 
The  cell  is  filled  by  pouring  through 
the  funnel  a  colored,  concentrated 
solution  of  cane  sugar.  The  stopcock 
is  then  closed  and  the  cell  placed  in  a 
beaker  of  distilled  water.  Although 
the  parchment  is  not  quite  imperme- 
able to  sugar,  it  will  be  seen  that  the 
water  passes  very  easily  through  the 
parchment  membrane  causing  a  rapid 
rise  of  the  solution  in  the  narrow  tube. 
The  initial  height  of  the  liquid  is 
marked  by  a  strip  of  paper.  The  rise 
amounts  to  several  centimeters  in  the 
course  of  an  hour. 

27.  Nernst2  has  constructed  an 
osmotic  cell  based  on  selective  solu- 
bility, similar  to  the  osmotic  gas  cell, 
described  above,  consisting  of  an  in- 
verted thistle  tube  (7  centimeters 
wide),  to  which  a  piece  of  pig  bladder, 
thoroughly  soaked  in  water  of  40°  is  tightly  fastened  by 
means  of  a  string.  The  cell,  filled  with  ether  in  which 
benzene  has  been  dissolved,  is  pressed  against  a  wire 
gauze,  suspended  in  a  400  cc.  beaker,  and  is  held  in  its 
place  by  a  clamp  from  a  ringstand  (Fig.  15).  The 
beaker  contains  ethyl  ether,  saturated  with  water,  and  is 
covered  by  a  one-hole  stopper  (allowing  the  passage  of 

1  Alex.  Smith,  1.  c.  p.  328. 

2  Zeitschr.  f.  phys.  Chem.,  6,  p.  37,  (1890). 


Fig.  14. 


28 


DEMONSTRATIONS    IN    PHYSICAL   CHEMISTRY 


the  stem)  in  order  to  limit  the  loss  of  ether  by  evapora- 
tion. The  wire  gauze  serves  to  prevent  the  bladder 
from  distending  as  a  result  of  the  passage  of  ether 
through  the  bladder  into  the  thistle  tube.  The  water  con- 
tained in  the  bladder,  dissolves  the 
ether,  but  not  the  benzene  and  acts  in 
this  manner  as  a  semipermeable  mem- 
brane. A  rise  of  10-20  centimeters  in 
the  course  of  an  hour  will  be  observed, 
provided  the  stem  of  the  thistle  tube 
is  not  too  wide. 

28.  In  a  very  striking  manner  Crum 
Brown1  has  illustrated  the  role  of  a 
perfectly  semipermeable  membrane. 
A  strong  solution  of  calcium  nitrate 
is  shaken  with  a  little  phenol,  until 
saturation  is  reached  and  the  mixture 
poured  into  a  high  and  narrow  cylin- 
drical jar.  The  phenol, left  undissolved, 
floats  as  a  thin  layer,  (which  should  not 
be  more  than  a  few  millimeters  thick) 
on  top  of  the  calcium  nitrate  solution, 
saturated  with  phenol.  The  phenol- 
layer  is  then  cautiously  covered  with  a 
saturated  solution  of  phenol  in  distilled  water.  The  cal- 
cium nitrate  being  insoluble  in  phenol,  the  latter  acts  as 
a  semipermeable  membrane  dissolving  the  water  and 
allowing  its  passage  from  the  upper  layer  into  the  lower, 
and  the  result  is, — as  a  daily  observation  and  demarca- 

1  Proc.  of  the  Royal  Soc.  of  Edinburgh,  22,  p.  439,  (1898). 


Fig.  15. 


OSMOSIS 


29 


tion  of  the  height  of  the  thin  phenol  layer  by  means  of 
strips  of  paper  shows, — that  the  phenol  is  gradually  dis- 
placed upward  until  finally  only  2  layers  are  left:  a 
dilute,  calcium  nitrate  solution,  surmounted  by  a  thin 
layer  of  phenol. 

The  experiments  with  three  liquid  layers,  of  which 
the  middle  acts, — to  a  certain  extent — 
as  an  osmotic  membrane,  were  first  car- 
ried out  by  a  French  scientist,  Lhermite.1 
As  liquids  he  used  an  aqueous  solution 
of  alcohol  (35  per  cent.),  castor  oil  (or 
turpentine)  and  water;  also  ethyl  ether, 
water  and  oil  of  bitter  almonds  (or  car- 
bon disulphide). 

29.  A  modification  of   one  of   Lher- 
mite's    three    liquid    combinations,    was 
introduced   by    Kahlenberg,2   who    fixes 
the  middle  layer  (in  this  case  water)  in 
its  position,  in  order  to  be  able  to  demon- 
strate   the    osmotic    pressure,    resulting 
from  the  difference  in  solubility  of  ether 
in  water  and  carbon  disulphide.    A  glass 
apparatus    (Fig.    16),    contains    at    the 
bottom  and  in  the  communicating  narrow 
side  tube,  mercury.     The  latter  is  cov-          **«•  16- 
ered  by  a  layer  of  carbon  disulphide  (C),  then  follows 
a  tightly  pressed  cork-disk  (B),  soaked  with  water,  and 
finally   a   layer    of    (aqueous)    ethyl    ether    (A).      The 
apparatus  is  closed  by  a  loosely  fitting  cork  to  avoid 

1  Ann.  de  Chim.  et  Phys.  (3)  43,  p.  420,  (1855). 

'2  Outlines  of  Chemistry,  Revised  ed.,  New  York,  p.  443,  (1916). 


3O  DEMONSTRATIONS    IN    PHYSICAL   CHEMISTRY 

evaporation  of  the  ether.  The  initial  position  of  the 
mercury  in  the  narrow  gauge  tube  is  marked  by  a  strip 
of  paper.  The  gradual  rising  of  the  mercury  may  be 
still  better  observed  by  pouring  some  colored  water  on 
to  the  mercury  in  the  side  tube. 


CHAPTER  IV. 


VAPOR  PRESSURE  AND  DETERMINATION  OF 
MOLECULAR  WEIGHTS. 

A.   Vapor  Pressure. 

Of  the  two  methods  for  measuring  vapor  pressures, 
the  static  and  the  dynamic,  the  former  is  easily  carried 
out  as  follows  :x 

30.  In  three  out  of  four  barometer  tubes,  all  inverted 
over  mercury,  are  inserted, — by  means  of  pipettes,  with 
their  ends  curved  upward — small  quantities  of  water, 
ethyl  alcohol  and  ethyl  ether  respectively.  The  fourth  is 
kept  as  a  standard  showing  the  atmospheric  pressure.  The 
fall  of  the  mercury  in  the  three  tubes  as  compared  with 
the  height  of  the  mercury  pile  in  the  fourth  tube  is  a 
direct  measure  of  the  vapor  pressure  of  the  liquids  at 
room  temperature.  Expressed  in  centimeters  of  mercury 
at  20°,  these  pressures  are: 

for  water  1.74 

for  alcohol     4.40 

for  ether    44.24 

By  mounting  the  tubes  within  a  jacket,  connected  with 
a  distilling  flask  containing  a  suitable  distilling  liquid, 
the  vapor  pressure  can  be  demonstrated  for  any  desired 
temperature. 

The  dynamic  method,  which  consists  in  a  slow  but  con- 
tinuous diminution  of  pressure  while  keeping  the  liquid 
constantly  at  ebullition,  allows  the  pressure  and  tempera- 

1  Bigelow,  Theor.  and  General  Chemistry,  New  York,  p.  274,  (1914). 


DEMONSTRATIONS    IN    PHYSICAL,   CHEMISTRY 


/     Z    3    4 


ture  to  be  read  at  the  same  time;  but,  owing  to  the  fact 
that  only  the  decrease  in  pressure  can  be  made  visible  to  a 
large  audience,  this  method  is  less  fit  for  a  lecture  dem- 
onstration. 

31.  The  vapor  pressure  of  solutions  is  shown  in  the 

same  way  as  for  pure  liquids,  viz., 
by  the  use  of  four  barometer  tubes 
over  mercury,  of  which  one  is  kept 
as  standard  of  comparison.  Into  the 
vacuum  of  the  first  tube  is  intro- 
duced a  little  ethyl  ether,  in  the 
second  is  placed  a  few  drops  of  a 
solution  of  12.2  grams  benzoic  acid 
in  100  cc.  ether  (molecular  weight 
of  benzoic  acid  =  122)  and  in  the 
third  tube  an  ethereous  solution  of 
benzoic  acid  of  double  this  strength 
(22.4  grams  in  100  cc.  ether).1 
After  a  while  the  difference  in 
mercury  level  between  tube  one 
and  two  is  about  half  as  much  as 
that  between  tube  one  and  three  showing  that  the  lower- 
ing of  the  vapor  pressure  is  proportional  to  the  concen- 
tration of  the  solute.  (Fig.  17.) 

32.  The  depression  of  the  vapor  pressure  at  the  boil- 
ing point   (under  atmospheric  pressure)   of  the  solvent, 
is  directly  connected  with  the  ebullioscopic  methods  for 
the  determination  of  molecular  weights  and  is  conven- 
iently   carried    out    with    the    aid    of    an    apparatus    as 

1  lyiipke-Bose,  Grundziige  der  Electrochemie,  5e  Aufl.  p.  106. 


17- 


VAPOR  PRESSURE;  AND  MOLECULAR  WEIGHTS 


33 


sketched  in  Fig.  iS,1  consisting  of  an  outer  jacket  and 
an  inner  "test"-tube  (  to  which  a  narrow 
gauge  tube  has  been  sealed),  held  by  a  two- 
holed  cork  stopper,  provided  with  a  second 
tube  for  the  escape  of  the  vapor  of  the  solv- 
ent. Pure  solvent  is  poured  in  the  outer 
jacket  and  also  in  the  test-tube,  to  a 
height  of  7  centimeters  above  the  bend.  In 
order  to  make  any  difference  in  level  better 
visible,  a  trace  of  some  aniline  dye  is  added. 
On  gently  heating  the  solvent  in  the  outer 
jacket,  the  vapor  condenses  along  the  walls 
and  finally  escapes  in  the  open  through  the 
outlet  tube.  The  latter  is  then  closed  by  a 
cork  stopper  and  the  vapor  is  forced 
through  the  narrow  gauge  tube,  expelling 
the  remaining  air,  and  condensing  above  the 
constricted  part  of  the  test-tube.  After  the 
vapor  has  bubbled  through  for  a  few  min- 
utes, the  cork  stopper  above  the  constric- 
tion is  pushed  down,  and  at  the  same  time 


Fig.  18. 

the  stopper  removed  from  the  outlet  tube,  thus  allowing 
the  vapor  to  escape  directly  in  the  open  as  before.  It 
will  be  found  that  the  level  is  practically  the  same  in  the 
test-tube  and  the  gauge  tube.  A  weighed  quantity  of  a 
solid,  easily  soluble  in  the  chosen  solvent  (about  0.3 
gram)  is  introduced  in  the  inner  tube,  and  the  operation 
is  repeated  When  equilibrium  is  reached,  the  liquid 
stands  lower  in  the  gauge  tube  (2  centimeters  or  more) 

1  Journ.  Am.  Chem.  Soc.,  40,  p.  193,  (1918).' 


34 


DEMONSTRATIONS    IN    PHYSICAL   CHEMISTRY 


than  in  the  test-tube,  thus  clearly  showing  that  the  pure 
solvent  has  a  higher  vapor  pressure  than  the  solution  at 
the  same  temperature.  On  adding  the  same  quantity  of 
solute  once  more,  the  fall  in  level  in  the  gauge  tube  will 
be,  after  equilibrium  is  re-established,  approximately 
twice  as  much  as  before.  A  suitable  solvent  for  a  lecture 
demonstration  is  carbon  tetrachloride,  on  account  of  its 
low  boiling  point  (76°).  its  non-inflam- 
mability, and  its  low  surface  tension  (the 
capillary  ascension  being  negligible)  .  As 
solute  naphthaline  or  any  other  organic 
compound,  which  dissolves  readily  in 
this  solvent  may  be  used. 

3,  Determination  of  Molecular  Weights. 
33.  The  above  described  method  of 
heating  the  solution  by  means  of  the 
vapor  of  the  solvent  has  been  used  by 
several  investigators  for  the  determin- 
ation of  molecular  weights.  In  cases 
where  it  is  only  necessary  to  decide  what 
multiple  of  the  empirical  formula  re- 
presents the  molecular  weight,  the  use  of 
a  modified  Landsberger  apparatus  as  the 
one  devised  by  Eykman1  (Fig.  19),  or 
a  similar  one,  by  McCoy,2  allows  a  mol- 
ecular  weight  determination  to  be  carried 
out  with  an  accuracy  of  5-10  per  cent,  in  the  course  of  a 
few  minutes.  For  a  lecture  demonstration  a  weighed 

1  Journ.  de  Chimie  Physique,  2,  p.  47,  (1903). 

2  Am.  Chem.  Journ.,  23,  p.  353,  (1900);  obtainable  from  Eimer  and  Amend, 
New  York. 


19. 


VAPOR    PRESSURE   AND    MOLECULAR   WEIGHTS 


35 


quantity  of  naphthalene  preferably  in  the  form  of  a  tablet 
(0.3  gram)  is  introduced  into  the  inner  tube  of  the 
Landsberger  apparatus  containing  12-16  cc.  of  benzene. 
The  outer  jacket  is  filled  with  about  50  cc.  of  the  solvent. 
The  thermometer,  on  which  the  boiling  point  is  read,  need 
not  be  a  "Beckmann;"  one  graduated  in  tenths  of  a  de- 
gree is  quite  sufficient  for  this  purpose.  As  a  matter 
of  course,  the  reading  of  the  boiling  point  and  the  volume 
of  the  solution  can  only  be  made  by  the  lecturer  or  his 
assistant. 

34.  The  vapor  density  method,  due  to  Victor  Meyer, 
is  applied  for  substances  which  can  be  readily  evapor- 
ated. A  description  can  be  found  in  any  textbook  on 
organic  chemistry.  Care  has  to  be  taken,  that  all  con- 
nections are  gas-tight  (thickwalled  India  rubber  tubing 
should  be  employed)  and  that  the  water  in  the  graduated 
glass  tube  is  saturated  with  air.  Using 
aniline  (boiling  point  184°)  in  the  outer 
jacket,  and  xylene  (ortho,  meta  or  para, 
boiling  point  140°)  in  the  glass-stop- 
pered weighing  bottle,  about  20  cc.  of  air 
will  be  collected  from  a  weight  of  about 
o.i  gram  of  liquid. 

Instead  of  glass-stoppered  weighing 
bottles  it  is  more  convenient  to  use  small 
glass  bulbs  with  a  sealed  capillary  stem, 
bent  at  the  end,  so  that  by  a  short  pull 
of  the  copper  wire  from  which  the  bulb 
is  suspended  the  stem  breaks  off  and  the  bulb  falls  in 
the  cylinder  below.  The  arrangement  is  readily  under- 


Fig.  20. 


DEMONSTRATIONS    IN    PHYSICAL   CHEMISTRY 


stood  from  Fig.  20.  A  side  tube  for  the  passage  of  a 
glass  rod  as  the  Meyer-apparatus  usually  contains,  is 
then  unnecessary. 

35.  The  cryoscopic  method  is  conveniently  carried  out 
with  the  Eykman  depressimeter  (Fig.  21).  This  simple  ap- 
paratus1 consists  of  a  short  thermom- 
eter, divided  into  twentieths  of  a  de- 
gree and  fitted  at  the  end  in  the  neck 
of  a  small  flask   (contents  about  20 
cc.),  fastened  inside  a  glass  cylinder 
by  means  of  a  cork  stopper  at  the  top 
and  a  plug  of  cotton  at  the  bottom. 
The  thermometer,  a  modified  "Beck- 
mann"  has  to  be  "set"  before  use.    A 
weighed  quantity  of  the  solvent,  say 
water    (10  grams)    is  poured  in  the 
flask  and  the  freezing  point  is  read. 
A  quantity  of  solute  of  known  weight 
is  then  added  and  the  freezing  point 
again  determined.       Taking   for  ex- 
ample 0.2  gram  of  sodium  chloride, 
the  depression  will  be  found    to    be 
about    1.30°,    instead  of  a  calculated 
value  (for    undissociated    molecules) 
of  0.65°. 

36.  Whenever  it  is  desired  to  make  the  readings  vis- 
ible to  the  auditory,  the  use  of  a  large  air  thermom- 
eter is  unavoidable.    Ciamician  has  proposed  the  follow- 
ing arrangement.2    A  cylindrical  glass  reservoir  (Fig.  22) 

*  Zeitschr.  f.  phys.  Chem.,  2,   p.  964,  (1888). 
2  Ber.  d.  chem.   Ges.,  22,  p.  31,   (1889). 


Fig.  21. 


VAPOR   PRESSURE   AND    MOLECULAR   WEIGHTS 


37 


is  sealed  to  a  glass  tube  (inner  cross  section  1.5  milli- 
meters), twice  bent  at  right  angles  and  provided  with  two 
bulbs  at  a  distance  of  50-60 
centimeters.  The  lower  part 
of  the  glass  tube  is  inserted  in 
an  alcoholic  solution  of  fuch- 
sine,  serving  as  a  confining 
liquid.  Around  the  glass  cyl- 
inder a  copper  stirrer  moves 
in  a  large  test-tube  (20  by  3 
centimeters).  This  test-tube 
is  filled  successively  with 
equimolecular  solutions  o  f 
cane  sugar  (34.2  grams  in  100 
cc.  of  water),  mannite  (10.2 
grams  in  100  cc.)  and  acetic 
acid  (16  grams  in  100  cc.) 
and  after  inserting  the  tube 
each  time  in  a  freezing  mix- 
ture of  salt  and  ice  the  freez- 
ing point  is  determined.  In 
all  three  experiments  the  con- 
fining liquid  rises  to  about  the  same  height. 


CHAPTER  V. 


CHEMICAL  EQUILIBRIUM  AND  THE  LAW  OF 
MASS  ACTION. 

Since  the  publication  of  Van  't  Hoff's  epochmaking 
"Etudes  de  dynamique  chimique"  (1884)  chemical  equi- 
librium and  the  law  of  mass  action  have  become  the 
nucleus  of  modern  physical  chemistry.  The  frequent 
and  manifold  applications  of  these  fundamental  prin- 
ciples, in  analytical  chemistry  and  elsewhere  make  it  de- 
sirable, to  take  up  the  discussion  of  this  subject,  directly 
after  the  properties  of  gases,  liquids  and  solids  have  been 
expounded.  It  goes  without  saying,  that  lecture  experi- 
ments in  this  field  especially,  can  only  illustrate  the  gen- 
eral laws  in  a  qualitative  way;  nevertheless  they  may 
be  considered  of  great  use,  in  clearly  demonstrating  the 
effect  of  concentration,  temperature  and  pressure  on  the 
course  of  chemical  reactions.  Taking  up  first  of  all,  the 
question  of  reversibility,  a  few  typical  reversible  reac- 
tions are  mentioned,  then  the  concentration  of  the  react- 
ing substances  is  considered  more  in  detail,  followed  by 
a  demonstration  of  the  change  of  equilibrium  by  varying 
temperature  or  pressure.  In  connection  herewith,  the 
effect  of  the  factors,  which  influence  the  velocity  of 
chemical  reactions  is  illustrated.  Finally  some  space  is 
devoted  to  the  rule  of  successive  reactions. 

Thus  the  chapter  may  be  subdivided  under  the  fol- 
lowing headings : 

I.  Reversible  reactions. 
II.  The  law  of  mass  action. 


CHEMICAL    EQUILIBRIUM    AND    MASS   ACTION  39 

III.  Displacement  of  equilibrium. 

IV.  Time  reactions. 

V.  Velocity  of  chemical  reactions. 
VI.  The  rule  of  successive  reactions. 

I.  Reversible  Reactions. 

In  spite  of  the  prevailing  tendency  in  analytical  chem- 
istry, of  carrying  out  reactions  along  "irreversible"  lines, 
it  has  been  recognized  in  the  past  decades,  that  in  prin- 
ciple every  reaction  is  reversible,  and  that  it  is  only  a 
question  of  choosing  the  proper  conditions  in  order  to 
revert  the  course  of  a  reaction.  Out  of  the  great  many 
cases  at  our  disposal,  the  following  examples,  easily  per- 
formed, may  be  quoted : 

37.  BiCls-f  H2O  ;F±  BiOCl  +  2HC1. 

To  a  small  quantity  of  bismuth  trichloride  in  a 
conical  lecture  jar  a  few  cubic  centimeters  of  5N  hydro- 
chloric acid  are  added,  until  a  clear  solution  is  obtained. 
When  water  is  added,  hydrolysis  occurs  and  a  white  pre- 
cipitate is  formed,  which  redissolves  on  the  addition  of 
concentrated  hydrochloric  acid. 

38.  Sb2Ss  -f  6HC1  ^  2SbCl,  +  3H2S. 

Concentrated  hydrochloric  acid  in  a  separatory  fun- 
nel is  allowed  to  drop  slowly  on  red  antimony  sul- 
phide in  a  fractionating  flask,  and  the  escaping  gas  passed 
into  a  solution  of  antimony  chloride,  forming  a  precipi- 
tate of  red  antimony  trisulphide. 

4 


40  DEMONSTRATIONS   IN    PHYSICAL   CHEMISTRY 

39.  H2S  displaces  CO2  from  its  salts. 

That  hydrogen  sulphide  can  displace  carbon  diox- 
ide from  its  salts,  just  as  well  as  the  latter  can  liberate 
hydrogen  sulphide  from  its  salts,  was  shown  by  Emil 
Fischer1  as  follows :  A  strong  current  of  hydrogen 
sulphide  is  passed  into  a  solution  of  sodium  bicarbonate 
thereby  liberating  carbon  dioxide.  The  gas  mixture,  bub- 
bling through  a  barium  hydroxide  solution  precipitates 
barium  carbonate.  The  reverse  takes  place  by  passing 
carbon  dioxide  into  a  solution  of  sodium  hydrosulphide ; 
the  hydrogen  sulphide  evolved,  precipitates  lead  sulphide 
from  a  solution  of  lead  acetate. 

40.  CH8COOH  displaces  CO,  from  its  salts. 

In  the  same  way  acetic  acid  (dilute)  displaces  carbon 
dioxide  from  a  solution  of  potassium  carbonate,  while  on 
the  other  hand  carbon  dioxide  passed  into  a  solution  of 
potassium  acetate  in  absolute  alcohol,  produces  a  precip- 
itate of  potassium  carbonate,  the  acetic  acid,  set  free,  re- 
maining in  solution.2 

41.  2H2O  ^  2H2  +  O2. 

The  decomposition  of  water  and  its  re-formation 
from  the  resulting  2:1  mixture  of  hydrogen  and  oxygen 
can  easily  be  shown,3  by  fastening  a  coil  of  platinum 
wire  (0.6  millimeter  in  diameter)  to  stout  copper  wires, 

1  Heumann-Kuhling,   Anleitung  zum  Experimentieren,    3e   Aufl.  p.  95, 
(1904). 

2  I^e  Cha teller,  logons  sur  le  carbone,  p.  210,  (1908). 

*  Hofmann,  Ber.  d.  chem.  Ges.,  23,  p.  3303,  (1890),  also  I^ash  Miller  and 
Kenrick,  Journ.  Am.  Chem.  Soc.,  22,  p.  296,  (1900). 


CHEMICAL   EQUILIBRIUM    AND    MASS   ACTION  4! 

passing  air-tight  through  small  thick-walled  glass-tubes 
(Fig.  23)  held  by  a  cork,  which  closes  the  neck  of  a  frac- 
tionating flask.  Water  is  boiled  in  the  flask,  until  the 
vapor  escapes  free  from  air;  the  platinum  wire  is  cau- 
tiously heated  electrically  to  white  heat,  by  connecting 
the  copper  wires  with  a  strong  current  cable  provided 


Fig.  23. 

with  ampere  meter  and  a  rheostat  (for  currents  up  to 
20  amperes).  The  arrangement  resembles  Deville's 
cold-hot  tube  in  bringing  about  a  dissociation  of  the 
water  vapor.  The  gas  mixture,  formed  in  the  reaction 
is  collected  over  water  in  a  eudiometer  and  then, — after 
passing  the  spark — recombined  to  water. 


42  DEMONSTRATIONS   IN    PHYSICAL   CHEMISTRY 

II.   The  Law  of  Mass  Action. 

As  illustrations  of  the  law  of  mass  action  several  in- 
structive lecture  experiments  have  been  devised.  Those 
given  below  have  been  found  to  be  most  convenient. 

42.  FeCl3  +  3NH4CNS  ^  Fe(CNS)3  +  3NH4C1. 

This  reaction,  which  was  first  systematically  in- 
vestigated by  Gladstone,1  may  be  carried  out, — following 
the  directions  of  L,ash  Miller  and  Kenrick2 — as  follows : 

Approximately  equivalent  solutions  of  ferric  chloride 
and  ammonium  thiocyanate  are  prepared,  the  first,  con- 
taining 6  grams  of  commercial  ferric  chloride,  25  cc.  of 
concentrated  hydrochloric  acid  (specific  gravity,  1.175) 
and  water  to  make  up  200  cc. ;  the  second  solution  con- 
tains 7.5  grams  of  ammonium  thiocyanate  dissolved  in 
200  cc.  of  water.  Five  cc.  of  each  solution  are  mixed  and 
2  liters  of  (tap)  water  added.  The  orange-colored  mix- 
ture is  equally  divided  between  four  beakers  of  600  cc. 
each.  From  the  color  of  the  four  solutions  it  is  evident 
that  the  equilibrium  is  considerably  displaced  to  the  left, 
ferric  thiocyanate  [Fe(CNS)3]  being  dark  red  in  so- 
lution; ferric  chloride  is  more  or  less  yellow,  while  am- 
monium salts  are  colorless.  Therefore,  the  amount  of 
ferric  thiocyanate,  present  in  solution,  can  be  fairly  well 
judged  from  the  depth  of  color  of  the  solution. 

On  adding  to  the  first  beaker  5  cc.  ammonium  thiocy- 
anate solution  and  to  the  second  5  cc.  ferric  chloride  so- 
lution the  color  becomes  in  both  cases  dark  red,  showing 
an  equilibrium  displacement  from  left  to  right  (»-*). 

1  Phil,  Trans.  Royal  Soc.,  p.  179,  (1855). 

2  Journ.  Am.  Chem.  Soc.,  22,  p.  292,  (1900). 


CHEMICAL    EQUILIBRIUM    AND    MASS   ACTION  43 

An  addition  of  50  cc.  of  a  saturated  solution  of  ammon- 
ium chloride  to  the  third  beaker  makes  this  solution  al- 
most colorless,  thereby  showing,  in  accordance  with  the 
law  of  mass  action,  that  the  equilibrium  is  now  displaced 
from  right  to  left  (  «—*).  The  fourth  beaker  is  kept  for 
comparison. 

43.  Ag-  +  Fe"  —  Ag  +  Fe"'. 

This  reaction  was  recently  studied  in  detail  by  A. 
A.  Noyes  and  Brann1  and  its  equilibrium  conditions  (at 
25°)  determined  carefully.  As  a  lecture  experiment 
this  reaction  may  be  performed  in  the  following 
manner  :- 

Pure  powdered  ferrous  sulphate  (FeSO4/H2O)  is  dis- 
solved in  cold  water  (previously  boiled,  to  drive  out  the 
air),  to  which  a  few  drops  of  sulphuric  acid  have  been 
added.  The  liquid  is  then  quickly  filtered  and  kept  in  an 
Erlenmeyer  flask  with  a  coil  of  thin,  rust-free  iron  wire. 

A  second  solution  is  made  by  adding  a  solution  of 
sulphuric  acid  (specific  gravity  1.25)  to  a  nearly  satu- 
rated solution  of  ferric  ammonium  alum  [Fe(NH4) 
(SO4)2i2H2O]  until  the  solution  is  almost  colorless,  or 
slightly  yellow.  The  addition  of  sulphuric  acid  serves  to 
hinder  hydrolysis.3  Five  to  ten  cc.  of  a  dilute  silver  ni- 
trate solution  are  then  poured  into  a  conical  lecture  jar 
and  mixed  with  enough  ferrous  sulphate  solution  to  form 
a  precipitate  of  silver.  The  latter  is  redissolved  after  the 
subsequent  addition  of  enough  ferric  alum  solution 

1  Ibidem,  34,  p.  1016,  (1912). 

2  I^uther,  Die  chemischen  Vorgange  in  der  Photographie,  Halle  p.  35  (1899). 
8  Ostwald,  Grundliuien  der  anorg.  Chem.,  2«  Aufl.,  p.  594,  (1904). 


44  DEMONSTRATIONS    IN    PHYSICAL    CHEMISTRY 

(about  four  to  six  times  the  quantity  of  the  ferrous  sul- 
phate solution,  required  for  precipitating  the  silver). 

In  connection  with  this  experiment  it  is  interesting  to 
point  out  the  mechanical  conceptions  by  which  Luther1 
and  later  on  Van  't  HofP  and  Baur2  have  tried  to  illus- 
trate the  chemical  equilibrium  in  this  reversible  reaction. 
The  former  imagines  a  balance  beam  (Fig.  24)  kept  in 
its  position  by  wire  coils  Fe"  and  Ag*  combining  their 
efforts  at  opposite  ends  in  one  sense,  while  Ag  and  Fe'" 


re"  ton 


coils  act  in  the  reverse.  More  appealing  to  the  mind  is 
the  idea  of  Van  't  Hoff  and  Baur,  as  represented  in  Fig. 
25.  By  plotting  the  percentage  composition  as  abscissa 
against  the  energy  of  the  system  as  ordinate,  we  see  that 
like  a  rolling  ball,  reaching  from  whatever  side  of  a 
curved  line  it  comes  down,  the  lowest  level,  the  energy 
of  the  system  reaches  a  minimum  value.  Similar  views 
have  been  expressed  by  L,eveing4  in  1885. 

1  I^uther,  1.  c.  p.  35. 

2  Baur,  Themen  der  physik.  Chemie,  lyeipzig,  p.  6,  (1910). 

»  Van  't  Hoff,  Physical   Chemistry    in  the   Service  of  the   Sciences,  p.  88, 
Chicago,  (1903). 

*  I^eveing,  Chemical  Equilibrium,  Cambridge,  p.  35,  (1885). 


CHEMICAL    EQUILIBRIUM    AND    MASS   ACTION  45 


/oo  re 
ore" 

Percentage   composition 

Fig.  25. 

44.  HgCl  +  OH'  ^±  HgOH  +  Cl'. 

Von  Dieterich  and  Wohler1  propose  the  reversible 
reaction,  expressed  in  the  above  equation  as  another  suit- 
able illustration  of  mass  action.  Phenolphthalein  being 
used  as  indicator,  a  N/ioo  solution  of  potassium  hy- 
droxide shaken  with  calomel  remains  red;  this  means  an 
incomplete  consumption  of  hydroxyl  ions ;  if  instead  of  a 
N/ioo  solution  a  N/iooo  solution  is  used,  the  color  of 
the  solution  changes  from  red  to  grey  on  shaking  with 
calomel ;  an  addition  of  a  few  drops  of  potassium  chlor- 
ide restores  the  original  red  color. 

1  Zeitschr.  f.  anorg.  Chem.,  34,  p.  194,  (1901). 


46  DEMONSTRATIONS   IN    PHYSICAL   CHEMISTRY 

in.  Displacement  of  Equilibrium. 

45.  The  change  of  equilibrium  by  lowering  or  raising 
the  temperature,  can  be  easily  shown,  in  dealing  with  gas- 
eous mixtures,  e.  g.,  nitrogen  tetroxide,  partly  dissociated 
in  nitrogen  dioxide : 

N2O4  ^  2N02. 

The  thermal  equilibrium  displacement  can  be  made  vis- 
ible in  this  case  first  by  the  accompanying  change  in 
color,  and  secondly  by  the  abnormal  change  in  pressure 
at  constant  volume. 

Taking  first  two  glass  tubes  (size  10  by  1.5  inches) 
filled  with  the  carefully  dried  gas  mixture  (prepared  by 
heating  lead  nitrate)  and  closed  at  both  ends  by  round- 
ing off  the  ends  in  the  blast  flame,  one  is  lowered  into  a 
cooling  mixture  of  alcohol  and  carbon  dioxide,  while 
the  other  tube  is  carefully  heated — with  proper  precau- 
tions, by  moving  the  flame  of  a  Bunsen  burner  along 
the  tube.  The  result  is  then  shown  by  placing  both  tubes 
simultaneously  against  a  white  background. 

46.  For    the    second    experiment,    two    round-bottom 
flasks  of  exactly  the  same  size   (contents  600-800  cc.) 
with  air-tight  fitting  ground  glass  stoppers,  to  which  U- 
shaped  open  manometer  tubes  (inner  bore  2  millimeters) 
have  been  sealed,  are  filled,   i  or  2  hours  in  advance, 
with  carbon  dioxide  and  nitrogen  tetroxide  respectively. 
Care  has  to  be  taken  that  the  stoppers,  carrying  the  man- 
ometer tubes,  are  well  attached  to  the  necks  of  the  flasks, 
so  as  to  hold  an  excess  pressure  (Fig.  26).    At  the  out- 
set both  manometers  show  the  same  pressure,  the  gases 
being   under   atmospheric    pressure.      On   inserting   the 


CHEMICAL    EQUILIBRIUM    AND    MASS   ACTION 


47 


flasks  held  by  clamps,  to  the  same  depth  in  a  large  water 
bath  of  50°,  the  manometer  of  the  nitrogen  tetroxide 
flasks  held  by  clamps,  to  the  same 
depth  in  a  large  water  bath  of  50°, 
the  manometer  of  the  nitrogen  tetra- 
oxide  flask  indicates  a  much  higher 
pressure  than  the  second  manometer, 
due  to  an  equilibrium  displacement 
from  left  to  right  (—»).  When 
taken  out  of  the  bath,  the  difference  in 
pressure  gradually  decreases,  until  fi- 
nally the  initial  state  is  reached. 

47.  The  effect  of  heating  on  gaseous 
dissociation  can  also  be  demonstrated 
in  the  reaction : 

PBrs  -f  Br2  ^  PBr5. 

Here,  too,  the  progress  of  dissocia- 
tion is  directly  visible  by  a  more  in- 
tense color,  since  on  raising  the  tem- 
perature, the  equilibrium  is  displaced 
from  right  to  left  (  •«-«• ) .  By  using 
equimolecular  quantities  in  one  tube  Fi«-  26- 

and  an  excess  of  phosphorus  tribromide  in  another  tube, 
the  same  experiment  also  illustrates  the  mass  action  law 
in  a  very  satisfactory  way.  Following  the  directions, 
given  by  Stieglitz,1  two  small  sealed  bulbs,  blown  at  the 
end  of  glass  capillaries,  containing  0.029  gram  bromine 
and  0.058  gram  phosphorus  tribromide  (a  little  more 
than  i  molecule)  respectively,  are  placed  in  a  piece  of 

1  Am.  Chem.  Journal,  23,  p.  404,  (1901). 


48  DEMONSTRATIONS   IN    PHYSICAL   CHEMISTRY 

thick-walled  glass  tubing,  closed  at  one  end  and  drawn 
out  at  the  other  end  into  a  capillary.  The  length  of  the 
tube  is  about  10  centimeters  and  its  capacity  40  cc.  The 
air  is  exhausted  to  20-30  millimeters  mercury  pressure, 
the  capillary  end  sealed  off  and  bent  into  a  loop.  By 
vigorous  shaking  the  bulbs  are  broken.  A  second  tube 
of  the  same  size  is  filled  in  the  same  way  with  a  mixture 
of  0.029  gram  bromine  (i  molecule)  and  0.45  gram  tri- 
bromide  (9  molecules). 

The  tubes  are  suspended  by  means  of  the  glass  loops 
at  the  upper  ends  in  a  tall  beaker  of  water  and  a  glazed 
white  porcelain  tile  or  a  piece  of  white  cardboard  placed 
behind  the  beaker,  to  make  comparison  of  colors  easier. 
On  heating  the  first  tube  is  slightly  colored  at  50°,  the 
second  not  at  all.  At  about  85°  the  most  favorable  stage 
for  comparison,  the  first  tube  appears  reddish  brown  and 
opaque,  while  the  latter  is  reddish  yellow,  through  which 
the  white  of  the  tile  or  cardboard  can  still  be  seen.  A 
similar  experiment  with  greater  differences  in  color  may 
be  carried  out,  using  phosphorus  trichlordibromide  with 
and  without  an  excess  of  the  trichloride. 

IV.   Time  Reactions. 

48.  The  fact,  that  certain  reactions  require  a  percep- 
tible time,  until  separation  of  one  of  the  reaction  prod- 
ucts starts,  is  illustrated  by  an  experiment,  performed  by 
Landolt,1  demonstrating  the  reduction  of  iodic  acid  by 
sulphurous  acid,  according  to  the  equation : 

5H,SO,  +  2HI03  =  5H,S04  +  H2O  +  I,. 

1  Ber.  d.  chem.  Ges.,  19,  p.  1317,  (1886)  ;  20,  p.  745,  (1887). 


CHEMICAL   EQUILIBRIUM    AND    MASS   ACTION  49 

Two  solutions  are  made  up,  one  of  1.8  grams  iodic 
acid  in  i  liter  water  and  the  other  composed  of  0.9 
gram  sodium  sulphite  (Na2SO37H2O),  5  grams  dilute 
sulphuric  acid  (1:10)  and  9.5  grams  soluble  starch 
(rubbed  to  a  thin  paste  by  adding  a  little  water)  in  I 
liter  water.1  These  stock  solutions  serve  to  make  up 
solutions  of  one-half  and  one-quarter  of  the  two  original 
concentrations.  On  mixing  rapidly  100  cc.  of  the  orig- 
inal and  of  the  more  dilute  solutions,  different  times 
will  be  found  before  noticeable  separation  of  iodine  sets 
in.  Sodium  sulphite,  being  easily  decomposed  in  contact 
with  air,  it  will  be  found  that  on  repeating  the  experi- 
ment, for  the  same  concentration,  not  necessarily  the 
same  time  as  before  is  registered. 

49.  More  reproducible  values  can  be  obtained  in  per- 
forming another  time  reaction,  also  studied  in  detail  by 
L,andolt:2  the  decomposition  of  thiosulphates  by  acids: 

H-  +  S2O8"  =  HSO8'  +  S. 
The  experiment  may  be  carried  out  as  follows: 
To  three  beakers  (of  200  cc.  each)  containing  respec- 
tively o.i,  0.2  and  0.3  gram  sodium  thiosulphate 
(NaaS2O35H2O)  100  cc.  distilled  water  is  added  and 
after  complete  solution  of  the  salt,  in  each  beaker  is 
poured,  at  the  same  time,  a  solution  of  i  cc.  concentrated 
hydrochloric  acid  in  20  cc.  distilled  water  (ready  at  hand 
in  three  test-tubes).  After  14,  6^  and  4  minutes  respec- 
tively a  milky  suspension  of  finely  divided  sulphur  be- 
comes visible.  In  a  fourth  beaker,  containing  0.2  gram 

1  H.  u.  W.  Biltz,  I.e.  p.  in. 

2  Ber.  d.  chem.  Ges.,  16,  p.  2958,  (1883). 


5<3  DEMONSTRATIONS    IN    PHYSICAL   CHEMISTRY 

thiosulphate  dissolved  in  100  cc.  distilled  water  and  kept 
at  50°  is  poured,  simultaneously  i  cc.  concentrated  hy- 
drochloric acid ;  the  result,  in  this  case  is  a  visible  sulphur 
separation  after  i%  minutes. 

It  is  interesting  to  notice,  that  the  different  investi- 
gators,1 who  studied  this  reaction  carefully,  all  agree  in 
admitting  an  immediate  formation  of  sulphur  on  mixing 
the  salt  and  acid  solutions.  The  sulphur  is  supposed 
to  remain  in  solution,  until  a  definite  concentration  is 
reached  or  a  certain  change  has  set  in,  which  causes  the 
appearance  of  visible  sulphur  drops. 

V.   Velocity  of  Chemical  Reactions. 

50.  That  the  rate,  at  which  a  chemical  reaction  takes 
place,  is  proportional  to  the  concentration  of  the  react- 
ing substances,  is  shown  by  the  following  experiment  of 
A.  A.  Noyes  and  Blanchard,2  referring  to  the  reaction, 
expressed  by  the  equation : 

HBr03  +  6HI  =  3H2O  +  3I2  +  HBr. 

Four  500  cc.  glass-stoppered  bottles,  8  centimeters  in 
diameter,  are  filled  with  400  cc.  dilute  hydrochloric  acid, 
made  up  by  mixing  1600  cc.  distilled  water  and  50  cc. 
N/2  hydrochloric  acid,  to  which  is  added  40  cc.  of  a 
starch  solution  (obtained  by  rubbing  i  gram  potato 
starch  with  5  cc.  cold  water  and  pouring  150  cc.  boiling 
water  over  it). 

From  four  10  cc.  graduates,  in  front  of  these  bottles, 

1  Holleman,  Rec.  d.  Trav.  Chim.  des  Pays-Bas  14,  p.  71,  (1895). 
v.  Oettingen,  Zeitschr.  f.  phys.  Chem.,  33,  p.  i,  (1900). 
Ostwald,  Grundlinien  der  anorg.  Cheinie,  3«  Aufl.,  p.  337,  (1912). 

2  Journ.  Am.  Chem.  Soc.,  22,  p.  739,  (1900). 


CHEMICAL   EQUILIBRIUM    AND    MASS   ACTION  51 

are  added  respectively  5,  10,  5  and  10,  cc.  of  a  N/2  solu- 
tion of  potassium  bromate,  and  then,  simultaneously — 
as  far  as  possible, — from  another  set  of  four  10  cc.  grad- 
uates, 5,  5,  10  and  10  cc.  respectively  of  a  N/2  solu- 
tion of  potassium  iodide.  The  glass  stoppers  are  quickly 
inserted  and  the  bottles  vigorously  shaken.  The  first 
mixture  will  become  of  the  same  shade  of  blue  as  a 
standard  starch-iodine  solution*  in  about  120  seconds, 
the  second  and  the  third  will  both  require  half  that  time, 
(about  60  seconds)  while  the  fourth  takes  on  the  color 
of  the  standard  solution  after  the  lapse  of  only  30 
seconds. 

51.  After  Nernst  and  Handa1  the  velocity  of  chemical 
reaction  can  clearly  be  demonstrated  by  saponifying 
methyl  formate,  the  progress  of  the  decomposition  of  the 
ester  being  shown  by  the  change  in  color  of  different 
indicators.  For  this  purpose  a  set  of  five  100  cc.  flasks 
are  filled  with  50  cc.  (previously  boiled)  water,  brought 
to  the  titer  of  N/iooo  with  barium  hydroxide.  The  fol- 
lowing indicators,  a  few  drops  in  each  case,  are  added : 
phenolphthalein,  litmus,  ganin,  p-nitrophenol,  and 
methyl  orange,  respectively.  From  five  small  test-tubes, 
all  attached  to  the  same  strip  of  wood, — in  order  to  in- 
sure a  simultaneous  action, — and  each  containing  I  cc.  of 
methyl  formate,  the  ester  is  poured  into  the  five  flasks. 
Besides  these,  another  set  of  five  flasks  without  addition 
of  ester  and  a  third  similar  set  of  five  flasks,  to  which 

*  The  standard  solution  kept  in  a  fifth  500  cc.  glass-stoppered  bottle  is 
made  by  adding  to  400  cc.  water  10  cc.  of  the  starch  solution  and  i  cc.  of  a  solu- 
tion of  i  gram  iodine  and  2  grams  potassium  iodide  in  500  cc.  water. 

1  Ber.  d.  chem.  Ges.,  42,  p.  3178,  (1909). 


52  DEMONSTRATIONS   IN    PHYSICAL   CHEMISTRY 

some  acid  has  been  added,  are  kept  for  comparison,  both 
sets  containing  the  indicators  in  the  above-mentioned 
order.  The  times  required  for  changing  the  color  of  the 
indicators  will  be  about  o,  I,  15,  30  and  120  minutes  re- 
spectively, and  give  an  idea  of  the  sensibility  of  the  in- 
dicators towards  OH'-ions. 

52.  The    influence    of    temperature    on    velocities    of 
chemical  reactions  is  preponderant  and  may  be  shown 
by  cooling  concentrated  hydrochloric  acid  and  a  piece  of 
marble  separately,  in  test-tubes  to  — 80°  in  a  mixture  of 
carbon  dioxide  and  alcohol,  and  then  bringing  both  to- 
gether by  dropping  the  marble  on  the  acid.    No  percept- 
ible gas  evolution  is  seen. 

53.  Even  comparatively  small  temperature  differences 
bring  about  considerable  changes  in  velocities  of  reac- 
tion.   An  instance  was  given  above  (page  49)  in  connec- 
tion with  the  decomposition  of  sodium  thiosulphate  by 
acids.    A  more  detailed  experiment,  illustrating  the  gen- 
eral rule,  that  equal  increments  of  temperature  cause  an 
equal  multiplication  of  the  velocity  of  any  chemical  re- 
action (roughly  speaking:  every  increase  of  temperature 
by  about  10°  doubles  the  velocity  of  the  reaction),  was 
given  by  Noyes  and  Blanchard.1 

The  reaction,  carried  out,  was  the  same  as  given  on 
page  50: 

HBrO,  4-  6HI  =  3H2O  +  3!,  +  HBr. 

A  solution  of  100  cc.  N/2  hydrochloric  acid  and  30  cc. 
starch  solution  in  noo  cc.  water  is  made  and  400  cc.  of 

1  Noyes  and  Blanchard,  1.  c.  p.  741. 


VAPOR  PRESSURE;  AND  MOLECULAR  WEIGHTS         53 

this  solution  poured  into  each  of  three  500  cc.  glass- 
stoppered  bottles  and  kept  in  three  water  baths  at 
4°,  16°,  and  28°*  respectively.  The  temperature  is  con- 
trolled by  a  large-size  thermometer,  inserted  in  succes- 
sion in  each  of  the  bottles.  Another  solution  is  made  up 
by  mixing  10  cc.  N/2  potassium  bromate,  10  cc.  N/2 
potassium  iodide  and  25  cc.  water.  Ten  cubic  centimeters 
of  this  mixture  is  placed  into  each  of  three  10  cc.  grad- 
uates. In  a  fourth  bottle  is  prepared  a  blue  iodine  starch 
solution,  in  order  to  serve  as  a  standard.  At  a  definite 
moment,  when  the  clock  (or  stop  watch)  shows  a  full 
minute,  the  three  bromate  solutions  are  poured  in  the 
three  bottles  and  then  after  being  stoppered,  vigorously 
shaken.  All  four  bottles  are  placed  against  a  white  back- 
ground and  the  times  noted,  at  which  the  three  solutions 
show  in  succession  the  standard  blue  color.  If  properly 
carried  out,  it  will  be  seen,  that  these  times  are  approxi- 
mately 32,  58  and  105  seconds.  Thus  a  rise  in  tempera- 
ture of  12°  and  24°  multiplies  the  velocity  of  the  reac- 
tion by  1.8  (58/32)  and  (i.8)2. 

54.  The  influence  of  pressure  on  the  velocity  of  chem- 
ical reactions  is  usually  of  no  great  importance.  One 
well-known  instance,  however,  where  pressure  has  a 
marked  effect,  may  be  quoted,  an  exception  also  to  the  ad- 
age of  the  ancients,  that  substances  do  not  react,  except 
when  dissolved,  viz.,  the  formation  of  mercuric  iodide : 

HgCl2  -f-  2KI  =  Hgl,  +  2KC1. 

*  The  temperature  of  the  third  solution  must  not  be  above  30°,  because  the 
depth  of  color  of  the  blue  solution  is  lessened  on  raising  the  temperature, 
owever,  not  perceptibly  below  30°. 


54  DEMONSTRATIONS   IN    PHYSICAL   CHEMISTRY 

A  mixture  of  powdered  potassium  iodide  and  corrosive 
sublimate,  in  equivalent  quantities,  shaken  in  a  wide- 
mouth  bottle,  is  only  slightly  colored  yellow,  due  to  the 
slow  formation  of  mercuric  iodide,  but  on  rubbing  the 
mixture  with  a  pestle  in  a  glass  mortar,  the  color  changes 
to  red,  owing  to  a  more  rapid  formation  of  the  red  iodide 
of  mercury. 

VI.   The  Rule  of  Successive  Reactions. 

That  many  reactions  take  place  with  the  temporary 
formation  of  less  stable  intermediate  compounds,  was  first 
observed  by  Gay-Lussac  and  as  a  result  of  many  observa- 
tions the  "law"  of  successive  reactions  was  introduced 
by  Ostwald1  and  simultaneously  by  Bancroft.  The  valid- 
ity of  this  "law"  taken  in  the  categorical  sense  in  which 
it  was  pronounced  by  these  two  investigators  has  been 
questioned  by  Nernst,  Bakhuis  Roozeboom,  Mellor  and 
others.  More  acceptable  is  the  formulation  of  this  prin- 
ciple (avoiding  the  much  abused  term  "law"),  proposed 
by  Alexander  Smith,2  stating,  that  "transformations, 
which  proceed  spontaneously  and  with  evolution  of  heat, 
may  go  forward  by  steps,  when  there  are  intermediate 
substances  or  allotropic  forms  capable  of  existence." 

55.  The  following  well-known  example  is  easily  car- 
ried out.  On  adding  stannous  chloride  to  a  solution  of 
mercuric  chloride  first  a  white  precipitate  of  calomel  is 
observed,  which  changes  after  a  while  (and  rapidly  on 
heating)  into  metallic  mercury. 

1  Zeitschr.  f.  phys.  Chem.,  22,  p.  306  (1897). 

2  1.  c.  p.  545. 


CHAPTER  VI. 

CATALYSIS. 

The  term  catalysis  was  introduced  in  chemistry  early  in 
the  1 9th  century  by  Berzelius.  The  importance  of  catalytic 
processes,  not  only  in  the  laboratory  but  also  for  indus- 
trial purposes  has  since  then  been  generally  recognized. 
A  recent  author  speaks  of  catalysis  as  a  chemical  "short- 
circuit."1  Ostwald  has  defined  this  phenomenon  as  a 
change  (mostly  increase)  of  velocity  of  chemical  reaction, 
by  the  addition  of  substances,  which  do  not  appear  in  the 
final  products  of  the  reaction.  This  definition  covers  a 
great  many  different  types  of  reactions,  which  are  all 
labelled  as  "catalytic"  and  which  may  be  distinguished, — 
following  the  classification  of  A.  A.  Noyes  and  Sam- 
met2  in  seven  types : 

1.  Reactions,     catalyzed    by     carriers     ("translation" 

agents), 

2.  Reactions,  catalyzed  by  absorbent  contact  agents, 

3.  Reactions,  catalyzed  by  electrolytic  contact  agents, 

4.  Reactions,  catalyzed  by  water, 

5.  Reactions,  catalyzed  by  dissolved  electrolytes, 

6.  Reactions,  catalyzed  by  enzymes, 

7.  Reactions,  catalyzed  by  inorganic  colloids: 

to  which  might  be  added  three  other  types : 

8.  Autocatalytic    reactions,    and    closely    related    with 

this  type, 

9.  Reactions,  with  intermediate  formation  of  catalytic 

agents,  and 

10.  Reactions,  catalyzed  by  "germs." 

1  Baur,  1.  c.  p.  62. 

2  Journ.  Am.  Chem.  Soc.,  24,  p.  498,  (1902). 

5 


56  DEMONSTRATIONS    IN    PHYSICAL   CHEMISTRY 

The  experiments  described  in  this  chapter,  are  ar- 
ranged according  to  these  ten  different  types.  Particulars 
concerning  most  experiments  described  below,  were 
taken  from  the  interesting  article  by  Noyes  and  Sammet, 
which  gives  complete  details  for  making  the  perform- 
ance as  easy  as  possible. 

TYPE  i. 

56.  Reaction  catalyzed : 

C6H6  -f  Br2  =  C6H5Br  +  HBr. 

Catalyzer:     ferric  bromide  (FeBr3). 

A  250  cc.  distilling  flask1  is  supported  upon  a  ring 
stand  and  its  side  arm  connected  with  the  stem  of  a  fun- 
nel, the  mouth  of  which  dips  just  below  the  surface  of  a 
potassium  hydroxide  solution.  In  the  flask  is  brought 
4  cc.  of  bromine  and  then  30  cc.  of  benzene  are  poured 
through  a  long-necked  funnel,  nearly  reaching  the  bot- 
tom of  the  flask.  No  reaction  occurs.  On  adding  0.5  cc. 
of  powdered  iron  and  after  blowing  some  gas  into  the 
neck  of  the  flask  from  a  small  wash  bottle,  containing 
strong  ammonia,  a  tight-fitting  cork  stopper  being  finally 
inserted, — great  clouds  of  white  fumes  are  seen,  escaping 
through  the  side  arm,  and  being  absorbed  in  the  caustic 
potash  solution. 

57.  Reaction  catalyzed : 

3C6H6  +  CHC13  =  CH(C6H5)3  +  3HC1. 

Catalyzer:    Aluminum  Chloride  (A1C13). 

This  is  the  type  of  reaction,  known  in  organic 
chemistry  as  the  synthesis  of  Friedel  and  Crafts.  The 
experiment  is  carried  out  by  pouring  in  a  test-tube  5-10 

1  Noyes  and  Sammet,  1.  c.  p.  501. 


CATALYSIS  57 

cc.  of  benzene,  to  which  a  few  drops  of  chloroform  are 
added.  No  reaction  takes  place,  not  even  on  gently 
heating  over  a  small  flame.  As  soon  as  a  small  quantity 
of  anhydrous  aluminium  chloride,  contained  in  a  small 
tightly  stoppered  tube  is  added,  a  copious  evolution  of 
dense  white  fumes  becomes  visible  and  at  the  same  time 
the  contents  of  the  test-tube  turns  dark  brown,  due  to  the 
formation  of  triphenylmethane.  The  reaction  has  to  be 
carried  out  under  a  glass  hood.  Its  success  depends  en- 
tirely on  the  quality  of  the  aluminium  chloride  used  for 
the  experiment.  Good  results  are  obtained  by  using  the 
granulated,  absolutely  dry  product,  manufactured  by 
Kahlbaum,  of  which  small  samples  are  to  be  kept  in 
well-corked  small  tubes,  ready  for  use.  Noyes  and 
Sammet  (1.  c.)  recommend  a  similar  reaction,  viz.,  the 
formation  of  acetophenone  from  benzene  and  acetyl 
chloride,  requiring  a  more  complicated  apparatus.  The 
arrangement  chosen  above  is  just  as  efficient  and  far 
more  simple,  since  only  a  test-tube  is  needed.  Other  re- 
actions of  this  type  may  be  looked  up  in  the  original 
paper.1 

TYPE:  2. 

58.  Out  of  numerous  examples,  belonging  to  this  type, 
sc.  catalysis  by  absorbent  contact  agents,  may  be  chosen 
the  reaction : 

2H2  +  02  =  2H20. 

Catalyzer :    Finely  divided  Platinum. 

A  mixture  of  hydrogen  and  oxygen,  in  the  propor- 
tion to  form  water  (about  10-15  cc.)  is  collected  over 
mercury  in  a  eudiometer  tube.  By  introducing  a  few 

1  Noyes  and  Sammet,  1.  c.  p.  499.502. 


58  DEMONSTRATIONS    IN    PHYSICAL   CHEMISTRY 

lumps  of  platinized,  granulated  pumice  stone  (prepared 
by  soaking  the  pumice  in  a  10  per  cent,  solution  of  chloro- 
platinic  acid  and  prolonged  heating  in  a  Bunsen  flame, 
until  the  platinum  is  left  in  a  finely-divided  state)  to  the 
mixture,  combination  takes  place,  demonstrated  by  a  de- 
crease in  volume  and  the  formation  of  a  water  nebula. 

59.  Another  instance  of  this  type  is  the  decomposition 
of  hydrogen  peroxide  by  the  catalytic  action  of  platinum 
black  and  bone  black.1 

2H2O2  =  2H3O  +  O2. 

In  each  of  two  lecture  test-tubes  is  placed  a  solution 
of  commercial,  concentrated  hydrogen  peroxide  (about  25 
cc.),  which  has  been  made  slightly  alkaline  by  the  addi- 
tion of  ammonia.  To  the  first  tube  is  added  i  cc.  of 
bone  black,  to  the  second  a  small  portion  of  platinum 
black.  In  both  tubes  a  violent  development  of  oxygen 
gas  takes  place  and  a  glowing  wood  splinter  inserted  in 
each  tube  rekindles.  The  required  platinum  black  is  pre- 
pared by  soaking  two  9  centimeter  filter  papers  in  a  10 
per  cent,  solution  of  chloroplatinic  acid  and  igniting  them 
in  a  large  porcelain  crucible,  until  the  carbon  is  burned 
off. 

TYPE;  3. 

60.  Most   electrolytic    catalyzers    accelerate    reactions 
through   the    formation   of   a   voltaic   couple.     A   well- 
known  case  is  the  increase  of  reaction  velocity  by  addi- 
tion of  one  drop  of  a  solution  of  chloroplatinic  acid  to  a 
pure  dilute  solution  of  sulphuric  acid,  in  which  a  sheet  of 
pure  zinc  is  inserted. 

1  Noyes  and  Sammet,  1.  c.  p.  504. 


CATALYSIS  59 

61.  A  similar  reaction1  is  the  following,  also  catalyzed 
by  platinum: 

Sn  +  2HC1  =  SnCl2  +  Hs. 

In  a  300  cc.  Erlenmeyer  flask,  provided  with  a  two- 
hole  rubber  stopper,  through  which  passes  a  thistle  tube 
and  a  delivery  tube  (ending  in  a  beaker  of  water)  is 
placed  a  layer  of  pure  feathered  tin,  covering  the  bottom 
of  the  flask  to  a  depth  of  2  centimeters.  On  pouring 
through  the  thistle  tube  enough  hydrochloric  acid  (specific 
gravity  1.12)  to  entirely  cover  the  tin,  only  a  slight  action 
occurs.  As  soon  as  a  little  chloroplatinic  acid  (or  copper 
sulphate  solution)  from  a  medicine  dropper  is  added,  a 
rapid  gas  evolution  occurs. 

TYPE  4. 

62.  Catalysis  by  water  is  shown  in  the  reaction : 

Zn  -f  I,  =  Znl,. 

Four  cubic  centimeters  of  powdered  iodine  are  placed 
into  a  test-tube  and  2,  cc.  of  zinc  dust  in  a  25  cc.  wide- 
mouthed  glass-stoppered  bottle.  The  iodine  is  poured 
into  the  bottle  and  the  mixture  vigorously  shaken. 
Nothing  visible  happens.  The  mixture  is  then  brought 
into  a  4  liter  glass  balloon,  supported  on  a  suitable  ring 
and  a  fine  stream  of  water  from  a  wash  bottle  directed 
on  the  dry  powder.  A  violent  action,  attended  by  a 
whizzing  noise  and  a  copious  evolution  of  iodine  vapors 
takes  place.2 

TYPE  5. 

Hydrolysis,  accelerated  by  an  acid,  in  virtue  of  its 
hydrogen  ions,  is  the  most  important  example  of  re- 

1  Noyes  and  Sammet,  1.  c.  p.  505. 

2  Noyes  and  Sammet,  1.  c.  p.  508. 


60  DEMONSTRATIONS    IN    PHYSICAL   CHEMISTRY 

actions  of  this  type.  Noyes  and  Sammet  suggest  that 
ions  are  mostly  hydrated  (already  verified  by  recent  in- 
vestigations) and  that  water  carried  as  hydrate  is  more 
active  than  ordinary  water.  If  this  suggestion  is  ac- 
cepted the  fifth  type  is  reduced  to  the  first  type,  mentioned 
above,  viz.,  that  of  carriers,  the  hydrogen  ions  acting  as 
water-carriers. 

63.  The  hydrolysis  of  starch,  catalyzed  by  sulphuric 
acid: 

(C6H1005)-  +  *H,0  =  *C6H1206, 
is  carried  out  as  follows1 : 

In  each  of  two  large  size  test-tubes  is  brought  an  equal 
weight  (i  gram)  of  starch.  To  one  of  the  tubes  is 
added  25  cc.  of  water,  to  the  other  25  cc.  of  a  5  per  cent, 
sulphuric  acid  solution.  Both  solutions  are  boiled  for 
about  half  a  minute.  The  acid  solution  is  neutralized 
with  9  cc.  of  a  50  per  cent,  caustic  potash  solution.  The 
contents  of  both  tubes  are  then  boiled  and  after  adding 
5  cc.  of  Fehling's  solution  to  each  tube,  both  are  boiled 
again.  It  will  be  seen  that  only  in  the  tube,  to  which 
acid  had  been  added,  a  red  precipitate  of  cuprous  oxide 
is  formed. 

TYPE  6. 

64.  An  experiment,  showing  the  catalysis  by  enzymes, 
is  the  hydrolysis  of  starch  solution  by  the  ptyalin,  present 
in  saliva : 

(C6H1006)*  +  *H20  ==  *C6H]206. 

In  a  test-tube  is  placed  a  small  quantity  of  starch, 
about  the  volume  of  a  split  pea.  Ten  cc.  of  water 
is  added  and  after  heating  to  boiling  10  cc.  more  of 

1  Noyes  and  Sammet,  1.  c.  p.  510. 


CATALYSIS  6l 

cold  water  is  added  and  then  2  or  3  drops  of  a  i  per  cent, 
iodine  solution.  The  liquid  turns  deep  blue,  which  color 
disappears  immediately  on  addition  of  25  cc.  of  fresh 
saliva.  Even  when  a  second  portion  of  iodine  solution  is 
added,  the  color  is  not  restored.1 

TYPE  7. 

65.  Inorganic  colloids  possess  strong  catalytic  power. 
Colloid  platinum  for  instance  greatly  accelerates  the  de- 
composition of  hydrogen  peroxide. 

2H2O2  ==  2H2O  -f  O2. 

A  colloid  platinum  solution,  prepared  according  to 
Bredig's  directions  (Chapter  IX,  p.  117)  is  used.  Placing 
25  cc.  of  hydrogen  peroxide  (commercial,  concentrated) 
in  each  of  two  lecture  test-tubes,  made  slightly  alkaline 
with  ammonium  hydroxide,  10  cc.  of  the  colloid 
platinum  solution  is  added  to  each.  To  the  first,  how- 
ever, has  been  added  previously  5  drops  of  a  saturated 
potassium  cyanide  solution.  The  difference  in  gas  evolu- 
tion in  both  tubes  is  striking.  A  vigorous  effervescence 
starts  in  the  solution,  which  contains  no  cyanide,  while 
in  the  other  solution  hardly  any  gas  evolution  occurs, 
thus  proving,  that  potassium  cyanide  acts  as  a  poison  in 
retarding  or  entirely  hindering  the  reaction. 

TYPE;  8. 

66.  A  case  of  autocatalysis  is  the  action  of  nitric  acid 
on    metals.2      A    sheet    of    pure    copper    or    silver    is 
inserted   in   a   lecture  jar,   filled   with   pure   nitric   acid 
(about  20  per  cent,  solution).       The  reaction  proceeds 

1  Noyes  and  Sammet,  1.  c.  p.  512. 

1  Ostwald,  Grundrisz  der  allgem.  Chemie,  4e  Aufl.,  Leipzig,  p.  336,  (1909). 


62  DEMONSTRATIONS   IN    PHYSICAL   CHEMISTRY 

very  slowly  until  after  a  while  a  vigorous  gas  evolution 
takes  place.  If  on  the  other  hand,  instead  of  pure  nitric 
acid,  fuming  nitric  acid,  containing  several  oxides  of  ni- 
trogen, is  taken,  an  immediate  solution  of  the  metal  will 
be  seen.  The  same  happens  on  addition  of  a  small 
amount  of  sodium  or  potassium  nitrite  to  the  pure  acid. 

67.  That  the  acceleration  of  the  velocity  of  reaction 
is  wholly  due  to  the  formation  of  lower  oxides  of  nitro- 
gen during  the  reaction  is  clearly  shown  by  an  inter- 
esting experiment  of  Quartaroli.1 

The  reaction,  studied  by  him  is  expressed  by  the 
equation : 

2KN03  +  6HCOOH  ==  N2O3  + 

4C02  -f  sH2O  +  2HCOOK. 

Taking  5  cc.  of  absolute  formic  acid,  heated  in  a  test- 
tube  at  40°,  to  which  is  added  0.3  gram  of  potassium 
nitrate,  the  reaction  sets  in  slowly,  but  after  2  minutes 
a  violent  gas  evolution  occurs,  which  is  finished  after  5 
minutes. 

The  same  experiment  is  performed  simultaneously  in 
another  test  tube,  to  which  I  milligram  of  potassium 
chlorate  has  been  added.  A  slight  retardation  is  per- 
ceptible. 

On  adding  3  milligrams  of  the  chlorate  to  a  third  tube, 
containing  as  before  5  cc.  of  formic  acid  and  0.3  gram  of 
the  nitrate,  a  visible  gas  evolution  takes  place  after  about 
10  minutes. 

To  a  fourth  tube,  is  added  5  milligrams  of  chlorate; 

1  Gazz.  chim.  ital.  41,  (II),  p.  64,  (1911). 


CATALYSIS  03 

no  reaction  at  all,  not  even  after  half  an  hour.  Thus  a 
small  amount  of  a  less  active  substance  is  able  to  hamper 
or  even  to  completely  paralyze  the  catalytic  action  of  a 
more  virulent  agent.  The  potassium  chlorate  may  be 
called  a  negative  catalyzer  in  regard  to  the  nitrogen  triox- 
ide,  the  catalyzer  formed  in  the  course  of  the  reaction. 

Reactions  of  this  "autocatalytic"  type  are  called  byBaur1 
"fever  reactions/'  owing  to  the  strong  resemblance  which 


Time 


Fig.  27. 


they  show  with  the  fever  process  in  the  human  body.  A 
period  of  scarcely  perceptible  reaction  (incubation  stage) 
is  followed  by  one  of  ever  increasing  velocity  (induction 
stage)  and  finally  by  a  decrease  in  activity,  generally 
more  rapid  than  the  foregoing  increase  (period  of  ex- 
tinction), as  is  graphically  represented  in  Fig.  27,  where 

1  Baur,  1.  c.  p,  66. 


64  DEMONSTRATIONS    IN    PHYSICAL    CHEMISTRY 

the  velocity  of  the  reaction  is  plotted  against  the  time  as 
abscissa. 

TYPE  9. 

A  similar  behavior  is  shown  by  reactions,  in  which  the 
catalyzing  agent  acts  temporarily,  being  formed  and  de- 
stroyed again  during  the  reaction.  The  same  periods  of 
incubation,  induction  and  extinction  can  be  distinguished, 
as,  for  instance,  in  the  reduction  of  potassium  permanga- 
nate by  oxalic  acid.1 

68.  The  reaction  takes  place  in  three  stages: 

I    Mnvn  +  V,  CA"  ->  Mn"  +  5CO2  (incubation). 
II     Mn™  +    4   Mn11    —  5Mnin  (induction). 

Ill    Mn111  +  V,  CA"  -~  Mn11  +  CO2. 

The  experiment  is  carried  out  in  four  200  cc.  beakers, 
the  first  of  which  contains  a  solution  of  10  cc.  N/io  potas- 
sium permanganate  in  no  cc.  of  water,  serving  as  a  stand- 
ard. In  the  second  is  placed  a  mixture  of  10  cc.  N/io 
potassium  permanganate  solution,  10  cc.  saturated  oxalic 
acid  solution  and  10  cc.  of  water.  The  third  beaker  and 
the  fourth  have  the  same  contents  as  the  second,  with  the 
addition  of  one  drop  of  manganous  chloride  (or  sul- 
phate) solution  to  the  third  and  of  an  excess  of  the  same 
reagent  to  the  fourth  beaker.  A  temporary  brown  col- 
oration becomes  visible  in  all  three  solutions,  but  with 
largely  different  velocity. 

69.  A  reaction  belonging  to  the  same  type  is  the  cataly- 

1  Baur,  1.  c.  p.  68. 


CATALYSIS  65 

sis    of    hydrogen  peroxide    by    chromic    acid,   studied 
by  Spitalsky.1 

The  reaction  is  performed  by  carefully  heating  in  a 
test-tube  at  4O-5O°C.  20  cc.  of  a  20  per  cent,  solution  of 
hydrogen  peroxide  (Merck's  solution),  to  which  has 
been  added  5  cc.  of  a  N/ioo  solution  of  chromic  acid. 
The  reaction  starts  slowly,  the  solution  becoming  blue ; 
after  about  10  minutes  a  violent  reaction  takes  place,  the 
color  of  the  solution  changing  into  red-violet  (induc- 
tion period).  Finally  the  reaction  slackens  and  comes  to 
a  stop,  and  the  original  color  of  the  chromic  acid  is  re- 
stored. 

TYPE  10. 

70.  The  part  played  by  so-called  "germs"  in  catalysis 
is  illustrated  by  an  experiment,  due  to  Luther,2  and  de- 
scribed by  him  as  follows : 

Some  word  is  written,  with  an  alum  crystal,  on  a 
clean  glass  plate.  Invisible  minute  crystals  remains, 
where  the  crystal  has  been  passed.  On  pouring  a  super- 
saturated alum  solution  over  the  glass,  crystallization 
starts  at  the  "germs"  and  the  word  becomes  visible. 

71.  As  a  final  experiment  on  catalysis  a  case  may  be 
quoted,  studied  by  Bredig  and  Wilke,3  which  shows  the 
periodic   character   of    some   catalytic   reactions.      In   a 
test-tube  is  brought  a  mixture  of  3.3  cc.  of  hydrogen 
peroxide    (the   authors   use    Merck's    "perhydrol"),   6.7 

1  Zeitschr.  f.  anorg.  Chem.  56  p.  72,  (1908). 

2  1.  c.  p.  24, 

»  Verh.  des  Naturhist,  med.  Vereins  Heidelb,  N.  F.  8,  p.  165,  (1905). 


66  DEMONSTRATIONS    IN    PHYSICAL   CHEMISTRY 

cc.  of  water  and  33  cc.  of  concentrated  sodium  acetate  so- 
lution. A  rather  small  drop  of  pure  mercury  is  added, 
and  after  a  while  a  periodic  gas  evolution  becomes 
visible,  due  to  the  alternate  formation  and  decomposition 
of  a  bronze  colored  peroxide  coat  on  the  mercury  drop. 


CHAPTER  VII. 


ELECTROCHEMISTRY  AND  IONIC  THEORY. 

Arrhenius'  theory  of  electrolytic  dissociation  (1887) 
has  such  an  important  bearing  on  the  science  of  electro- 
chemistry that  a  joint  consideration  of  both  is  nowadays 
a  matter  of  course,  the  one  being  inseparably  connected 
with  the  other.  Numerous  instructive  lecture  demonstra- 
tions illustrating  the  present  conceptions  on  this  subject 
have  been  devised  by  various  physico-chemists.  In  the 
selection,  chosen  below,  a  review  of  the  material  at  hand 
is  made  under  the  following  headings : 

I.  Electrolysis. 
II.  Migration  of  ions. 

III.  Electromotive  chemistry. 

IV.  Conductivity  and  degree  of  ionization. 
V.  The  common  ion  effect. 

VI.  Hydrolysis. 

VII.  Ionization  and  chemical  activity. 
VIII.  Ionization  and  color  of  solutions. 

I.   Electrolysis. 

72.  Experiments  on  electrolysis  of  salt  solutions  and 
fused  salts  are  so  well  known,  that  a  special  description  at 
this  place  seems  superfluous.  Familiar  demonstrations  in 
elementary  chemistry  courses  are :  the  electrolysis  of  cop- 
per sulphate  solutions  between  platinum  and  between  cop- 
per electrodes,  also  of  sulphuric  acid,  usually  carried  out 
in  a  Hofmann  apparatus,  formerly  frequently  called  "ap- 
paratus of  electrolysis  of  water,"  and  of  potassium  sul- 


DEMONSTRATIONS    IN    PHYSICAL   CHEMISTRY 


phate  solutions.  The  electrolysis  of  stannous  chloride 
and  of  lead  acetate  is  interesting  on  account  of  the  for- 
mation of  tin  and  lead  "trees"  and  is  for  projection  pur- 
poses conveniently  performed  in  small  glass  troughs  with 
parallel  walls,  using  small  metal  rods,  running  through 
a  cork,  as  electrodes. 

73.  In  order  to  obtain  the  easily  decomposable  alkali 

metals  in  the  form  of  amal- 
gams Nernst1  has  devised  the 
following  arrangement : 

A  large  test-tube  (12  by  1.5 
centimeters)  connected  with  a 
capillary  outlet  tube  (Fig.  28), 
contains  mercury,  covered  by 
a  layer,  about  3  centimeters 
thick,  of  chloroform  and  an- 
other layer  of  concentrated 
potassium  chloride  solution. 
The  tube  is  closed  by  a  three- 
hole  cork  stopper,  allowing 
the  passage  of  (a)  a  glass 
tube  for  the  escape  of  gases 
during  the  electrolysis,  (b)  a 
strong  platinum  wire  with 
'  v  spiral  windings  and  (c)  a 

*»••'  funnel  of  about  25  cc.  capac- 

Flg-  28>  ity,  drawn  out  into  a  capillary 

tip,  and  filled  with  mercury.  A  platinum  wire  is  inserted 
in  the  mercury  and  both  wire  electrodes  connected 

1  Zeitschr.  f.  Electrochemie  3,  p.  308,  (1897). 


ELECTROCHEMISTRY   AND   IONIC    THEORY 


with  a  battery  of  three  lead  accumulators.  The  mercury, 
dropping  into  the  solution  forms  potassium  amalgam,  pro- 
tected from  the  decomposing  action  of  water  by  the 
chloroform  layer  and  collects  in  the  beaker  placed  under 
the  outlet.  On  pouring  water,  containing  a  few  drops 
of  phenolphthalein  into  the  beaker,  the  liquid  instantly 
turns  red  and  evolution  of  hydrogen  becomes  visible. 

n.   Migration  of  Ions. 

74.  As  introductory  to  lecture  experiments,  showing 
migration  of  ions,  the  change  in  concentration  near  the 
electrodes  may  be  demonstrated. 

A  small  glass  trough  (Fig.  29),  with  parallel  walls, 
as  used  for  projection  purposes,  is 
filled  with  a  dilute  copper  sulphate 
solution  acidified  with  some  sul- 
phuric acid.  Two  L-shaped  copper 
wires  are  inserted  and  connected 
with  the  poles  of  a  storage  cell. 
After  a  while  it  will  be  seen  on  the 
screen  that  the  blue  color  at  the 
cathode  brightens,  whereas  a  more 
concentrated  solution  collects  near  the  anode.1 


29« 


75.  A  similar  experiment  was  devised  by  Palmaer.2 
A  U-shaped  glass  tube,  about  70  centimeters  high,  with 
an  inner  bore  of  1.5  millimeters,  filled  with  a  4N  solu- 
tion of  hydrochloric  acid  is  used.  A  silver  wire  is  used 
as  anode,  while  a  platinum  wire  serves  as  cathode;  both 

1  Coehn  in  Miiller-Pouillet's  Handbook  IV,  p.  493,  (1909). 

2  Zeitschr.  f.  Electrochemie  12,  p.  513,  (1906). 


DEMONSTRATIONS    IN    PHYSICAL   CHEMISTRY 


are  inserted  as  far  as  the  middle  of  the  limbs  (Fig.  30). 
The    electrodes    are    connected 


with  a  loo-volt  circuit.  The 
current  is  about  0.02  ampere  at 
the  start,  but  sinks  in  the  course 
of  the  electrolysis  to  about  0.015 
ampere,  the  silver  wire  being 
gradually  covered  with  a  thin 
layer  of  silver  chloride.  The 
difference  in  level  amounts  to  4 
millimeters  after  5  minutes,  and 
increases  on  further  passing  the 
current  through  the  solution. 

A  silver  anode  is  employed  in 
order  to  avoid  an  increase  of 
specific  gravity  by  dissolved 
chlorine.  The  wire  must  have 
been  used  several  times  since  the 

gas  is  not  easily  taken  up  by  a  polished,  uncorroded 

surface. 

76.  Lodge1  first  introduced  the  use  of  gelatin  jellies 
for  the  direct  measurement  of  ionic  velocities.  These 
jellies  may  be  safely  used  instead  of  pure  aqueous  solu- 
tions provided  the  percentage  of  gelatin  does  not  exceed 
4-5  per  cent,  since  careful  investigations  have  brought 
out  the  fact  that  dissolved  salts  diffuse  through  gelatin 
jellies  at  about  the  same  rate  as  through  pure  water.  The 
method  of  Lodge  is  as  follows : 

A  graduated  glass  tube,  40  centimeters  long  and  8 

1  Brit.  Ass.  Report,  p.  393,  (1886)  ;  p.  389,  (1887). 


Fig.  30. 


ELECTROCHEMISTRY   AND   IONIC   THEORY 


millimeters  wide,  is  twice  bent  at  right  angles  and  the  end 
slightly  curved  upward,  as  shown  in  Fig.  31.    The  tube 


Fig.  31. 

is  rilled  with  a  solution  of  sodium  chloride  in  gelatin, 
made  up  by  dissolving  10  grams  of  gelatin  in  140  cc.  of 
hot  water,  and  adding  7  grams  of  salt  and  a  few  drops 
of  a  slightly  alkaline  solution  of  phenolphthalein.  This 
mixture  readily  gelatinizes  upon  cooling.  Both  ends  of 
the  tube  are  then  inserted  in  two  beakers,  containing  di- 
lute sulphuric  acid.  On  passing  a  current  through  the 
solutions  and  the  jelly, — applying  a  storage  battery  of  ten 
cells  (about  twenty  volts)  as  electromotive  force, — a 
gradual  decoloration  of  the  jelly  will  be  observed.  It  will 
be  seen  that  the  boundary  surface  moves  at  the  rate  of 
1.5  centimeters  in  i  hour. 

77.  In  the  particular  case  that  colored  ions  are  con- 
sidered, the  migration  is  easily  shown  by  means  of  a 
simple  apparatus,  originally  devised  by  Nernst1  and 
slightly  modified,  as  described  below : 

A  U-shaped  glass  tube,  1.2  centimeters  in  diameter  and 

i  Zeitschr.  f.  Electrochemie  3,  p.  308,  (1897). 

6 


DEMONSTRATIONS    IN    PHYSICAL    CHEMISTRY 


io  centimeters  high,  is  connected  in  the  lower  part  of 
the  bend  with  a  piece  of  capillary  glass  tubing  (length 
20  centimeters,  inner  bore  2  millimeters)  bent  upward, 
to  which  is  sealed  a  separatory  funnel  of  100  cc.  con- 
tents. The  solution  used  for  this  experiment  is  made 
up  by  dissolving  0.5  gram  of  potassium  permanganate  in 
100  cc.  of  distilled  water,  the  specific  gravity  of  which 
has  been  increased  by  the  addition  of  5  grams  of  urea. 
In  order  to  fill  the  capillary  tube, 
some  of  the  solution  is  poured  in 
the  bend  and  sucked  up  into  the 
funnel,  until  the  liquid  has  risen 
above  the  stopcock  which  is  then 
turned  off.  The  liquid  remaining 
in  the  bend,  is  rinsed  out  with  dis- 
">-N  tilled  water  and  the  latter  removed 
^  by  turning  the  U-tube  upside  down. 
71  j|7  The  funnel  is  then  filled  with  the 
rest  of  the  permanganate  solution. 
In  the  now  empty  U-tube,  is  poured 
by  means  of  a  io  cc.  pipette,  a 
solution  of  0.5  gram  of  potassium 
nitrate  in  I  liter  of  water.  Both 
limbs  are  closed  with  two-hole 
rubber  stoppers,  allowing  the  pas- 
sage of  two  platinum  wires,  pro- 
vided with  perforated  platinum 
electrodes,  and  of  two  small  glass 

tubes   for  the  escape  of  the  gases   evolved   during  the 
electrolysis.     On  carefully  opening  the  stopcock  the  per- 


ELECTROCHEMISTRY   AND   IONIC   THEORY  73 

manganate  solution  drives  the  colorless  nitrate  solution 
with  sharp  boundary  surfaces  into  the  limbs  of  the  U- 
tube  to  a  certain  height,  marked  by  a  white  and  a  black 
strip  of  paper  respectively  (Fig.  32). 

When  a  current, — not  exceeding  0.2-0.3  ampere — is 
passed  through  the  tube,  the  violet  boundary  surface  is 
gradually  displaced  in  the  direction  of  the  anode,  distinct- 
ly visible  after  about  5  minutes  The  current  is  turned  off 
as  soon  as  the  boundary  surface  on  the  anodic  side  be- 
comes irregular  owing  to  convection  currents.  In  the 
left  (cathodic)  limb,  the  boundary  surface  remains  ex- 
tremely sharp  and,  as  Nernst  has  pointed  out,1  the  migra- 
tion velocity  of  the  MnO4'-ion  can  be  approximately 
calculated  from  the  lowering  of  the  boundary  surface. 

78.  Kiister,2  with  the  aid  of  two  U-tubes,  as  described 
in  the  foregoing  experiment,  shows  how  copper  in  a  cop- 
per sulphate  solution  moves  towards  the  cathode  and  in 
Fehling's  solution  in  the  opposite  direction. 

The  bend  of  the  left  U-tube  is  filled  with  a  light  blue 
(dilute)  solution  of  copper  sulphate,  separated  by  a  sharp 
boundary  surface  from  a  dilute  sodium  sulphate  solution 
in  both  limbs.  The  second  U-tube  is  filled  in  an  exact- 
ly similar  way  with  a  dark  blue  Fehling's  solution  cov- 
ered in  both  limbs  by  a  dilute  alkaline  solution  of 
Rochelle  (Seignette)  salt.  After  inserting  the  platinum 
electrodes,  joined  in  parallel,  an  electric  current,  derived 
from  a  storage  battery  of  15-20  accumulators,  is  passed 
through  both  tubes.  After  5-10  minutes  the  copper  sul- 
phate boundary  has  moved  several  millimeters  towards 

1  Nernst,  1.  c.  p.  309. 

2  Zeitschr.  f.  IJlectrochemie  4,  p.  112,  (1898). 


74  DEMONSTRATIONS   IN    PHYSICAL   CHEMISTRY 

the  cathode,  while  in  the  other  tube  a  movement  in  oppo- 
site direction  has  taken  place,  a  sure  indication  that  in 
this  case  the  copper  forms  part  of  a  complex  anion. 

79.  Instead  of  an  apparatus  as  used  by  Nernst,  a  simple 
U-tube  (suitable  dimensions:  height  16  centimeters,  in- 
ner bore  2  centimeters)  will  serve  the  requirements,  when 
in  place  of  aqueous  solutions,  agar-agar  jellies  are  em- 
ployed,1 thus  returning  to  Lodge's  original  device. 

A  solution  of  agar-agar  is  first  made  by  cutting  25 
grams  of  this  substance  in  small  pieces  and  treating  with 
500  cc.  of  distilled  water.  The  mixture  is  then  heated 
until  a  clear  solution  is  formed,  which  is,  while  still 
hot,  strained  through  a  piece  of  cloth.  To  50  cc. 
of  this  hot  solution  is  added  about  10  cc.  of  a  saturated 
copper  sulphate  solution  and  this  mixture  poured  into 
the  U-tube  to  about  4  centimeters  above  the  bend  (Fig. 
33).  The  jelly  is  allowed  to  harden  and  a  little  bone 
black  sprinkled  on  the  surface  to  mark  the  boundary. 
In  order  to  fix  the  bone  black  in  its  place  a  solution  of 
potassium  nitrate,  saturated  at  o°,  containing  agar-agar 
is  poured  in  each  limb  of  the  tube,  and  after  hardening, 
the  tube  on  both  sides  filled  up  with  potassium  nitrate 
solution.  The  U-tube  is  placed  in  a  large  beaker  with 
ice  water,  to  preserve  the  jellies  from  melting  on  elec- 
trolyzing  the  copper  sulphate.  Electrodes  of  platinum 
wire  are  inserted  in  the  potassium  nitrate  solution  and 
connected  with  a  16  candle-power  lamp  in  series,  with 
the  terminals  of  a  no- volt  lighting  circuit.  On  passing 
the  current  through  the  tube  for  5-10  minutes,  the  effect 

1  A.  Smith,  I.e.  p.  346. 


ELECTROCHEMISTRY   AND    IONIC    THEORY 


75 


of  the  displacement  of  the  blue  boundary  surface  towards 
the  cathode  becomes  apparent.  The  movement  of  the 
colorless  SO4"-ions  can  be  demonstrated  by  interposing, 


Fig.  33. 

on  the  positive  side  a  thin  layer  of  jelly  containing  some 
barium  salt,  in  which  case  a  cloudy  layer  of  barium 
sulphate  jelly  is  formed. 

80.  A   similar   experiment  with  a   jelly   containing  a 
solution  of  copper  chromate  enables  the  demonstration 
of  the  simultaneous  movement  of  the  blue  copper  ion  and 
the  yellow  chromate  ion  in  opposite  directions   (Noyes 
and  Blanchard1). 

81.  The  relative  velocity  of  migration  of  different  ions 
can  be  demonstrated  in  an  instructive  experiment  given 

» 1.  c.  p.  729. 


76  DEMONSTRATIONS    IN    PHYSICAL   CHEMISTRY 

by  Noyes  and  Blanchard.1  Careful  determinations  have 
established  that  at  room  temperature  the  ionic  mobilities 
per  hour,  in  dilute  aqueous  solutions  for  a  potential  dif- 
ference at  the  electrodes  of  I  volt  amounts  to  2.05,  2.12, 
10.8,  5.6,  and  1.6  centimeters  for  the  ions  K', 
Cl',  H*,  OH'  and  Cu"  respectively.  Broadly  speak- 
ing, K*  and  Cl'-ions  move  at  the  same  speed, 
H'-ions  move  about  five  times  as  fast,  double  as  fast 
as  OH '-ions  and  eight  times  as  fast  as  Cu"-ions. 
Therefore,  it  is  advisable  to  use  a  potassium  chloride  so- 
lution in  which  these  different  ions  are  all  present,  Cu"- 
ions  being  visible  by  their  color,  H'-ions  being  recognized 
by  decoloration  of  phenolphthalein  and  OH'-ions  by 
coloring  this  indicator. 

The  bend  of  a  U-tube,  as  described  above,  and  the 
right  limb  (Fig.  34),  up  to  a  point  5  centimeters  from 
the  top,  is  filled  with  a  jelly  made  by  mixing  32  cc.  of 
saturated  potassium  chloride  solution,  i  cc.  of  a  I  per 
cent,  solution  of  phenolphthalein  in  alcohol,  100  cc.  of 
a  2  per  cent,  agar-agar  solution  and  8  drops  of  a  normal 
solution  of  potassium  hydroxide.  The  other  limb,  up  to 
5  centimeters  from  the  top,  is  filled  with  the  same  mix- 
ture, to  which  has  been  added  twice  the  amount  of  hydro- 
chloric acid,  necessary  for  decolorizing  the  liquid.  The 
boundaries  in  both  limbs  are  fixed  by  sprinkling  a  little 
bone  black  on  the  surfaces  and  covering  the  bone  black 
with  a  thin  layer  of  the  underlying  jelly  in  order  to 
keep  the  black  demarcation  surface  intact.  The  platinum 
wire  electrodes  are  placed  at  the  top  of  the  limbs  of  the 

i  1.  c.  p.  731. 


ELECTROCHEMISTRY   AND   IONIC   THEORY 


77 


U-tube  and  connected  through  a  32  candle-power  elec- 
tric lamp  with  the  terminals  of  a  no-volt  direct  current 
circuit.  Just  before  starting  the  experiment  the  left 
limb  is  filled  up  with  a  mixture  of  2  cc.  of  a  10  per 
cent,  potassium  hydroxide  solution  and  20  cc.  of  a  satu- 
rated potassium  chloride  solution;  the  other  arm  of  the 
tube  is  then  filled  up  with  a  mixture  of  0.5  cc.  of  hydro- 


Fig.  34. 

chloric  acid  (specific  gravity  1.12),  6  cc.  of  a  saturated 
copper  chloride  solution  and  20  cc.  of  water.  The  U- 
tube  is  placed  in  ice  water,  to  prevent  the  melting  of  the 
jelly  by  the  heat  generated  during  the  electrolysis.  On 
closing  the  switch  and  allowing  the  current  to  pass  for 
about  15  minutes,  it  will  be  observed,  that  a  colorless 
zone  (due  to  the  H'-ions)  descends  into  the  pink  jelly 
in  the  right  limb  to  a  depth  of  about  5-6  centimeters,  fol- 


78  DEMONSTRATIONS    IN    PHYSICAL   CHEMISTRY 

lowed  by  a  blue  zone  (accounting  for  the  Cu**-ions)  of 
about  i  centimeter  deep.  (See  Fig.  346.)  In  the  other 
arm  a  pink  zone  (due  to  the  OH'-ions)  descends  into  the 
colorless  jelly  to  a  depth  of  about  2.5  centimeters. 


Electromotive  Chemistry. 
A  large  number  of  reactions  involving  ionogens  are 
known,  in  which  chemical  changes  are  accompanied  by 
the  liberation  of  electrical  energy.  Since  all  these  ar- 
rangements for  obtaining  electric  currents  are  in  reality 
nothing  but  voltaic  cells,  this  special  branch  of  chemistry 
may  very  appropriately  be  designated  as  electromotive 
chemistry.1  The  essential  feature  about  the  combina- 
tions for  the  production  of  electric  currents  consists  in 
preventing  the  active  substances  from  coming  in  contact 
with  each  other.  This  can  be  done  in  different  ways,  as 
will  be  seen  from  the  following  interesting  lecture  ex- 
periments, mostly  suggested  by  Kiister  and  by  L,iipke. 

82.  The  first  type  of  cell  to  be  considered  is  the  "dis- 
placement cell."  Iron,  displacing  copper  from  its  solu- 
tion according  to  the  equation  : 

Cu"  +  Fe^  Cu  +  Fe". 

produces  a  current  in  the  connecting  wire,  running  from 
the  copper  to  the  iron,  as  indicated  by  the  arrow.  As  a 
current  indicator  for  this  and  the  following  demonstra- 
tions a  sensitive  lecture  galvanoscope  (of  Keiser  and 
Schmidt)  or  a  suitable  milli-ampere  meter  may  be  used. 
A  Weston  station  voltmeter,  in  which  the  series  resist- 
ance coil  has  been  short-circuited,  will  also  serve  the  pur- 

i  A.  Smith,  1.  c.  p.  786. 


ELECTROCHEMISTRY   AND    IONIC   THEORY 


79 


pose.    A  full  scale  deflection  is  obtained  with  a  current 
of  about  o.oi  ampere.    The  experi- 
ment is  carried  out  as  follows:1 

A  disk-shaped  copper  electrode 
and  a  polished  iron  rod  electrode 
are  inserted  in  a  large  crystalliza- 
tion dish  filled  with  a  solution  of 
sodium  sulphate  (Fig.  35).  On 
connecting  both  electrodes  with  the 
current  indicator,  no  current,  or  at 
least  no  lasting  current,  is  noticed. 
As  soon  as  some  solid  copper  sul- 
phate is  placed  on  the  copper  disk, 
thereby  surrounding  the  electrode 
with  Cu"-ions  a  strong  current  re- 

SultS'  Fig.  35. 

83.  In  the  same  way  the  reaction  : 

2Fe"'  -f  Fe  !^±  3Fe" 

will  give  an  electric  current,  in  the  direction  of  the  ar- 
row. The  same  apparatus  is  used  as  in  the  preceding  ex- 
periment, replacing  the  sodium  sulphate  solution  by  a  so- 
lution of  sodium  chloride  and  the  copper  electrode  by  a 
platinum  disk.2  No  perceptible  current  is  observed,  but 
on  bringing  some  solid  ferric  chloride  on  the  platinum 
disk,  which  is  thus  surrounded  by  Fe*"-ions,  a  current 
in  the  direction  of  the  arrow  results  (Fig.  36).  The 
process  that  takes  place  consists  in  discharging  the  tri- 


1  Kiister,  Zeitschr.  f.  Electrochemie  4,  p.  107,  (1897). 

2  Kiister,  1.  c.  p.  107. 


So 


DEMONSTRATIONS   IN    PHYSICAL   CHEMISTRY 


valent  iron  ion  and  the  simultaneous  loading  of  the  un- 
charged iron. 

84.  Another  way  of  producing 
an  electric  current  is  by  discharging 
a  cation  and  at  the  same  time  giv- 
ing another  cation  a  higher  charge : 

Sn"  +  Hg"  =  Sn—-f  Hg. 

An  apparatus,  as  devised  by 
Lupke1  may  be  used,  consisting  of 
two  beakers,  one  of  which  con- 
tains an  acidulated  solution  of 
stannous  chloride,  (112  grams  in  I 
liter),  while  the  other  is  filled  with 
an  acidulated  normal  sodium  chlor- 
ide solution.  The  beakers  are  con- 
nected by  a  wide  siphon,  filled 
Fig.  36.  with  the  same  solution  of  sodium 

chloride.  Platinum  electrodes,  bent  at  right  angles,  are 
inserted  in  the  beakers,  and  attached  to  copper  wires, 
leading  to  a  galvanoscope.  No  current  is  observed.  On 
placing  a  few  crystals  of  corrosive  sublimate  on  the  right 
electrode,  a  current  flows  through  the  wire  circuit  from 
right  to  left,  as  shown  by  the  galvanoscope.  (Fig.  37.) 

85.  Instead  of  cations,  anions,  may  be  used  to  furnish 
electricity  as  in  the  reaction : 

I'  +  Br  ^  Br'  +  I. 

In  a   H-shaped  vessel    (Fig.   38)    two  circular  plati- 
num foils  are  sealed  in  near  the  bottom,  and  the  connect- 

1  l,upke-Bose,  1.  c.  p.  164. 


ELECTROCHEMISTRY   AND    IONIC   THEORY 


8l 


ing  copper  wires  attached  to  a  galvanoscope.  A  10  per 
cent,  potassium  chloride  solution  is  poured  into  the  ves- 
sel and  the  platinum  disk  in  the  left  limb  covered  with 
a  few  drops  of  bromine.  No  current  is  observed,  but 


Fig.  37.  Fig.  38. 

on  placing  a  crystal  of  potassium  iodide  on  the  elec- 
trode in  the  right  limb,  the  galvanoscope  indicates  a  cur- 
rent in  the  wire  circuit  from  left  to  right.  At  the  same 
time  the  solution  on  the  left  side  is  colored  brown  by 
the  separation  of  iodine.1 

86.  The  reversible  ionic  reaction : 

Fe"    +  1°  ^  Fe-"  +  I', 

1  Kiister,  1.  c.    p.  109. 


82 


DEMONSTRATIONS    IN    PHYSICAL   CHEMISTRY 


in  which  both  cations  and  anions  take  part,  can  also  pro- 
duce an  electric  current. 

Following  again  Kuster's  directions,1  a  large  size 
crystallization  dish,  (Fig.  39),  filled  with  moderately  di- 
luted hydrochloric  acid,  is  used. 
Two  small  dishes  are  placed  in- 
side, so  that  the  liquid  covers 
both.  Platinum  foils,  bent  at 
right  angles,  are  inserted  in  each 
dish  to  serve  as  electrodes.  The 
platinum  foil,  on  the  left  i  s 
covered  with  some  iodine  crys- 
tals, the  other  by  a  few  pure 
ferrous  sulphate  crystals  (even- 
tually 2  or  3  drops  of  a  freshly 
prepared  ferrous  chloride  solu- 
tion). The  galvanoscope  shows 
a  current  flowing  through  the 
wire  from  left  to  right.  After  a 
while, — enough  Fe"'-ions  being 
formed, — the  current  can  be  re- 
verted by  adding  potassium  iodide  crystals  to  the  iodine 
electrode. 

87.  Another   reversible   ionic   reaction,   already   men- 
tioned in  a  preceding  chapter,  viz.  : 

Fe"  +  Ag-  =  Fe;"  +  Ag, 

can  be  adapted  to  give  a  current  of  electricity  in  the 
manner  described  by  L,ermontoff.2     The  experiment  is 

1  Id.,  1.  c.  p.  108. 

2  Meldola,  the  Chemistry  of  Photography,  I^ondon.p.  179,  (1891). 


Fig.  39. 


ELECTROCHEMISTRY   AND   IONIC   THEORY  83 

well  fitted  for  projection  on  the  screen  by  dividing  a  glass 
cell  with  parallel  sides  into  two  partitions  by  means  of 
a  piece  of  brown  paper  cemented  in  a  vertical  position, 
water-tight  to  each  side  and  to  the  bottom.  The  cell  is 
then  filled  on  one  side  with  a  2  per  cent,  solution  of 
silver  nitrate  and  on  the  other  with  a  cold  saturated  so- 
lution of  ferrous  sulphate.  On  connecting  both  solutions 
through  a  bent  silver  wire,  dipping  in  each  partition 
half  way  to  the  bottom,  a  crystalline  growth  of  silver  on 
the  wire  can  be  observed  on  the  side  which  contains  the 
silver  nitrate. 

The  above  mentioned  reactions  deal  with  the  type  of 
galvanic  cells,  called  "displacement  cells."1  Two  other 
types  are  the  "combination  cell"  and  the  "oxidation  cell." 

88.  A  combination  cell  may  be  set  up  for  instance  by 
taking  a  glass  vessel,  divided  in  two  partitions  by  a  por- 
ous diaphragm  (of  unglazed  porcelain)  and  filled  on  one 
side  with  a  sodium  chloride  solution,  in  which  a  zinc  rod 
is  dipped,  and  on  the  other  side  with  the  same  solution 
to  \vhich  some  bromine  has  been  added.    A  platinum  wire 
or  a  rod  of  carbon  is  inserted  in  this  solution  and  both 
poles  connected  with  copper  wires  to  a  galvanoscope.    A 
current  flows  through  the  wire  circuit  from  the  platinum 
(or  carbon)  to  the  zinc  and  the  reaction,  that  takes  place 
in  the  solutions  on  both  sides  of  the  septum  is  the  fol- 
lowing : 

Zn°  +  2Br°  =  Zn"  -f  2Br'. 

89.  The   same   arrangement   may   be  used    for   illus- 

»  A.  Smith,  1.  c.  p.  788. 


84 


DEMONSTRATIONS   IN    PHYSIC AI,   CHEMISTRY 


trating  the  operation  of  an  oxidation  cell.1  such  as  is  ex- 
pressed by  the  equation  : 

Sn"  +  2Cl°  =  Sn-"  +  2CF. 

Both  partitions  are  filled  with  the  same  sodium  chlor- 
ide solution,  and  then  some  stannous  chloride  dissolved 
in  the  left  hand  solution,  while  on  the  other  side  free 
chlorine  is  introduced.  Two  platinum  wires  are  in- 
serted and  connected  with  a  galvanoscope  which  in- 
dicates a  current  from  right  to  left. 

A  fourth  type  of  cell,  vis.  concentration  cells,  will  be 
considered  later. 

90.  The  different  "types"  of  gal- 
vanic cells  may  be  divided  in  two 
groups  :  inconstant  and  constant  cells. 
Thus  the  combination  Fe-NaCl-Pt 
(page  79)  e.  g.  is  an  inconstant  cell. 
The  ferric  chloride  added  to  the  plati- 
num electrode,  acts  as  a  "depolarizer." 
Liipke2  has  modified  the  apparatus,  as 
sketched  in  the  figure  (Fig.  40),  the 
ferric  salt  being  poured  on  the  plati- 

Ynum  disk  through  the  side  tube. 
91.  The  "polarization"  current  can 
ke  easily  demonstrated  by  electrolyz- 
ing  dilute  sulphuric  acid  (1:10)  in  a 
H-shaped  vessel  (Fig.  41),  communi- 
cating with  a  large  crystallization  dish,  filled  to  two- 
thirds  of  its  height  with  the  same  acid.  The  electrodes 
are  platinized  platinum  foils,  the  cathode  being  inserted 

1  Ibidem,  1.  c.  p.  792. 

2  Riidorff-Iyiipke,  Grundrisz  der  Chemie,  12e  Aufl.,  p.  306,  (1902). 


ELECTROCHEMISTRY   AND   IONIC   THEORY 


twice  as  deep  into  the  acid  as  the  anode.  The  acid  is 
electrolyzed  with  one  storage  cell  and  the  electrolysis 
continued  until  the  lower  end  of  the  platinum  foils  just 
touches  the  liquid  in  both  limbs.  The  current  is  then 
turned  off  and  connection  is 
made  with  a  galvanoscope,  which 
indicates  a  current  in  the  con- 
necting wire,  flowing  in  the  op- 
posite direction. 

The  electrodes  are  platinized 
by  placing  the  platinum  foils, 
previously  cleaned  by  means  of 
chromic  acid,  in  a  solution  of  3 
grams  of  platinum  chloride  and 
0.02-0.03  gram  of  lead  acetate  in 
100  cc.  water,  and  connecting 
the  electrodes  with  a  battery  of 
two  lead  accumulators.  The  cur- 
rent is  passed  for  10-15  minutes, 
reverting  its  direction  through  a 
commutator  every  half  minute.1 

92.  During  the  electrolysis  of 
dilute  acids  both  platinum  elec- 
trodes are  polarized.  On  adding  oxidizing  agents  to 
the  cathode,  the  evolution  of  hydrogen  is  stopped  and 
cathodic  polarization  prevented. 

This   is   clearly   shown  in  the   following  experiment, 
devised  by  L,iipke  :2 

Three    U-shaped    tubes    with    sealed    platinum    foil 

1  Findlay,  Practical  Physical  Chemistry,  p.  171,  (1915). 
-  Rudorff-IyUpke,  1.  c.  p.  305. 


Fig.  41. 


86 


DEMONSTRATIONS   IN    PHYSICAL   CHEMISTRY 


electrodes  (Fig.  42),  are  connected  in  series  with  a 
storage  battery  of  eight  lead  accumulators.  The  first 
tube  is  filled  with  a  19  per  cent,  nitric  acid  solution, 
the  second  with  a  52  per  cent,  nitric  acid  solution  and 
the  third  with  a  chromic  trioxide  solution.  As  soon  as 
the  current  is  turned  on,  it  will  be  noticed  that  oxygen 


Fig.  42. 

is  evolved  at  all  three  anodes;  hydrogen  is  only  set  free 
in  the  first  tube,  while  in  the  second  vapors  of  nitrogen 
oxide  escape.  In  the  third  tube  the  color  of  the  liquid 
turns  gradually  to  a  darker  shade. 

93.  A  well-known  inconstant  cell  is  the  combination 
Zn-H2SO4-Cu.  By  eliminating  polarization  a  constant  cell 
results,  as  is  proved  by  the  following  lecture  experiment.1 

1  Brauer,  lyehrbuch  der  anorg.  Chemie  2e  Aufl.,  p.  188,  (1913). 


ELECTROCHEMISTRY   AND    IONIC   THEORY  87 

The  funnel  A  (Fig.  43),  about  two-thirds  filled  with  di- 
lute sulphuric  acid  (i  no)  is  connected  by  means  of  rub- 
ber tubing  with  a  second,  leveling  funnel  (provided  with 
stopcock)  containing  a  copper  sulphate  solution.  A  con- 
ical sheet  of  zinc,  which  is  amalgamated  with  mercury 
in  order  to  minimize  the  direct  action  of  the  zinc  on  the 


Fig.  43. 


acid  acts  as  anode,  while  a  copper  disk,  farther  down  to 
the  bottom  serves  as  cathode.  On  connecting  the  elec- 
trodes with  a  low  resistance  ampere  meter,  the  latter  in- 
dicates right  at  the  start  a  current  of  about  1.5  ampere, 
rapidly  decreasing,  however,  to  0.2-0.3  ampere.  When 
the  copper  sulphate  is  allowed  to  flow  into  the  cell,  cov- 
7 


DEMONSTRATIONS    IN    PHYSICAL   CHEMISTRY 


ering  the  cathode  and  forming  a  "Daniell"  or  "gravity" 
cell,  the  current  increases  in  strength  and  becomes  con- 
stant. 

94.  Very  simple  in  construction  is  the  apparatus,  de- 
scribed by  lyiipke,1   which  enables   the  use  of   several 
depolarizers  in  succession. 

The  electrolyte,  dilute  sulphuric  acid,  (1:25)  is  con- 
tained in  a  narrow  mouth  bottomless  bottle  held  upside 
down  by  a  clamp  fastened  to  a  rin£  stand  (Fig.  44). 
The  cathode  is  a  large  copper  disk  sol- 
dered to  a  copper  rod,  passing  through 
the  cork  stopper.  The  anode,  a  platinum 
disk,  is  separated  from  the  cathode  by  a 
crystallization  dish.  On  connecting  the 
electrodes  with  a  galvanoscope,  no  ap- 
preciable current  is  indicated,  but  on 
pressing  crystals  of  potassium  perman- 
ganate, corrosive  sublimate,  silver  ni- 
trate, small  cubes  of  manganese  dioxide 
or  red  lead  on  the  platinum  disk,  the 
Fig.  44.  pointer  immediately  deviates. 

95.  Concentration  cells  are  cells  in  which  two  different 
concentrations  of  the  same  salt  are  used.    Such  a  cell  is 
readily  set  up  by  half  filling  with  a  concentrated  solu- 
tion of  copper  chloride  (20  grams  CuCl2  in  40  cc.  water), 
a  wide  glass  tube  (15  by  2.5  centimeters)  closed  at  its 
lower  end  by  a  one  hole  cork  stopper,  through  which 
passes  a  copper  rod.     On  top  of  this  is  poured  a  dilute 

i  Riidorff-I,upke,  1.  c.  p.  307. 


ELECTROCHEMISTRY   AND    IONIC    THEORY 


89 


solution  of  the  same  salt  (20  grams  in  I  liter  water), 
taking  care  that  a  sharp  boundary  surface  is  maintained. 
The  tube  is  closed  by  another  perforated  stopper  carry- 
ing a  copper  rod  dipping  in  the  dilute  solution  (Fig.  45). 
When  connection  is  made  with  a  galvano- 
scope,  the  pointer  indicates  a  current, 
ing — outside  the  tube, — from  the  lower 
rod  to  the  upper.1  Similar  concentration 
cells  may  be  constructed  with  Ag-AgNO3- 
and  Zn-ZnSO4-  solutions. 

96.  With  a  slight    modification    "short- 
circuited,"  concentration  cells  are    formed, 
first  described  by  Bucholz  in  i8c>4.2 

A  lecture  jar  (15-20  centimeters  high)  is 
filled  to  one-half  of  its  contents  with  a 
concentrated  solution  of  stannous  chloride, 
obtained  by  dissolving  15  grams  of  tin  in 
dilute  hydrochloric  acid  and  evaporating  to 
40  cc.,  and  this  solution  is  covered  with  a 
very  dilute  solution  of  the  same  salt.  A 
tin  rod,  inserted  in  the  jar,  so  that  it  passes 
through  both  layers,  is  partly  corroded  by 
dissolving  in  the  dilute  solution,  and  below  f* 
the  boundary  surface  covered  with  a  "tin 
tree."  (Fig.  46.) 

97.  An  experiment,   showing  that  shortcircuited  gal- 
vanic cells  are  possible,  which  are  entirely  built  up  of 
liquids,  has  been  devised  by  Kriiger  and  Dolezalek.3 

1  lyiipke,  Grundziige  der  Electrochemie,  5«,  Aufl.,  p.  144. 

2  Coehn,  1.  c.  p.  555. 

3  Zeitschr.  f.  Electrochemie  12,  p.  669,  (1906). 


DEMONSTRATIONS    IN    PHYSICAL   CHEMISTRY 


An  O-shaped  glass  vessel  (Fig.  47),  with  a  bore 
of  6  centimeters,  is  half  way  filled  with  a  35  per  cent, 
solution  of  sulphuric  acid,  colored  with  litmus.  On  the 
left  side  a  layer  of  sodium  acetate  solution  (30  per 
cent.),  2  centimeters  high,  is  placed.  In  order  to  obtain 


Fig.  46.  Fig.  47. 

a  sharp  boundary,  the  solution  is  cautiously  dropped 
from  a  pipette  on  a  thin  cork  disk,  floating  on  the  acid. 
The  ring  is  then  filled  up,  in  the  same  way,  with  a  20 
per  cent,  lithium  chloride  solution,  containing  a  few 
drops  of  ammonia,  colored  with  litmus.  The  ring  is 
brought  around  a  small  magnet  system,  so  that  the  lat- 
ter occupies  the  center  of  the  ring.  The  system  con- 


ELECTROCHEMISTRY   AND   IONIC   THEORY  $1 

sists  of  several  small  magnets,  suspended  from  a  wire, 
3  centimeters  long,  and  enclosed  in  a  thick-walled  cop- 
per box  (5  centimeters  high  and  2  centimeters  wide)  pro- 
vided with  a  small  glass  window.  A  mirror  is  fixed  on 
the  magnet  system,  which  allows  to  throw  the  image  of 
an  illuminated  arrow  on  a  graduated  screen  at  2-3  meters 
distance.  A  curved,  astatic  bar  magnet,  the  distance  of 
which  from  the  magnet  system  can  be  regulated,  is  placed 
underneath  the  ring,  in  order  to  increase  the  sensibility 
of  the  measuring  instrument,  which  indicates  a  slight 
current  flowing  through  the  ring.  By  turning  the  ring 
through  180°,  the  arrow  moves  in  the  opposite  direc- 
tion, over  the  same  number  of  scale  divisions  on  the  other 
side  of  the  zero-point.  Upon  shaking  the  ring  the  liquids 
become  mixed  and  the  arrow  returns  to  its  initial  posi- 
tion. The  experiment  may  be  taken  as  proof  that  Volta's 
law  does  not  hold  for  solutions. 


IV.    Conductivity  and  Degree  of  lonization. 

98.  Pure  water  does  not  conduct  an  electric  current 
perceptibly.  A  current  of  appreciable  strength  is  only 
noticed  by  dissolving  salts,  acids  and  bases  in  water. 
This  is  shown  by  filling  a  beaker  of  200  cc.  with  distilled 
water  and  inserting  two  platinum  foils  (3  by  4  centi- 
meters) parallel  to  each  other,  at  a  distance  of  1-2  centi- 
meters. On  connecting  the  electrodes  with  a  galvano- 
scope  and  a  battery  of  three  lead  accumulators,  no  cur- 
rent is  indicated;  but  on  allowing  concentrated  hydro- 
chloric acid  to  drop  from  a  pipette  into  the  water,  the 


92  DEMONSTRATIONS   IN    PHYSICAL  CHEMISTRY 

instrument  shows  a  constantly  increasing  deviation  from 
the  zero  point. 

99.  That,  on  the  other  hand,  it  is  not  the  acid  alone 
which  is  responsible  for  the  conductivity,  can  be  proved 
by  passing  dry  hydrochloric  acid  gas  into  carefully  pre- 
pared toluene,  from  which  all  traces  of  water  have  been 
removed.     On  inserting  two  platinum  electrodes,  con- 
nected with  a  galvanoscope  and  a  battery  of  70  volts, 
into  the  solution,  no  current  is  indicated.    A  few  drops 
of  water,  however,  immediately  have  the  effect  of  pro- 
ducing a  current  of  noticeable  strength.1 

100.  The  following  experiment,  due  to  Scriba,2  illus- 
trates the  same  fact  for  sodium  chloride.     Solid  rock 
salt,  like  pure  water,  does  not  perceptibly  conduct  elec- 
tricity, but  when  it  is  dissolved  in  water,  the  solution 
shows  itself  a  good  conductor.     A  glass  tube,  20  centi- 
meters long  and  with  a   diameter  of  2.5   centimeters, 
closed  at  one  end  and  provided  with  two  platinum  wires 
sealed  in  the  glass  at  the  lower  end  of  the  tube,  is  half 
filled  with  distilled  water.    A  cubical  piece  of  solid  rock 
salt  (if  necessary,  dried  with  absolute  alcohol)  is  fixed  be- 
tween two  brass  clamps  (Fig.  48).    Connection  is  made 
with  the  terminals  of  a  no- volt  direct  circuit,  a  switch 
and  a  32  candle-power  lamp  being  interposed,  so  that  the 
salt    and    water    are    in    parallel    in    the    circuit.      On 
closing  the  circuit,  no  electrolysis  is  observed  and  the 
lamp  does  not  glow,  but  on  dropping  a  small  piece  of  the 
rock  salt  in  the  water  the  lamp  gradually  shines  with  a 

1  Kiister,  1.  c.  p.  109. 

2  Zeitschr.  f.  phys.  u.  chem.  Unterricht  28,  p.  94,  (1915). 


ELECTROCHEMISTRY   AND    IONIC    THEORY 


93 


bright  yellow  light  and  electrolysis  takes  place  in  the  so- 
lution of  rock  salt.  The  experiment  may  be  repeated, 
replacing  the  rock  salt  by  a 
large  crystal  of  cane  sugar. 
The  result  in  this  case  is 
negative. 

101.  That  the  conductiv- 
ity of  a  given  weight  of 
electrolyte  increases  with 
increasing  dilution  is  readi- 
ly demonstrated  by  the  fol- 
lowing  experiment  of 
Stieglitz,1  adapted  from  a 
similar  one  by  Noyes  and 
Blanchard.2 

A  rectangular  glass 
trough,  of  about  i  liter 
capacity,  4.6  centimeters 
wide  11.5  centimeters  long 
and  20  centimeters  high  is 
fitted  with  copper  elec- 
trodes, 4.6  centimeters  broad  and  21  centimeters  high 
(Fig.  49)  connected  with  a  lead  accumulator  and  an  am- 
pere meter.  On  bringing  20  cc.  of  a  4N  hydrochloric 
acid  solution  in  the  trough,  the  current  registered  by  the 
amperemeter  will  be  after  a  few  seconds,  0.17  ampere. 
On  adding  successively  20,  40,  80,  160  and  320  cc.  of 
distilled  water,  the  mixture  being  well  stirred  after  each 

1  Qualitative  Analysis.  Vol.  1,  p.  49,  (1916). 

2  1.  c.  p.  726 ;    similar  experiments  have  been  described  by  I«upke  and 
by  Ostwald. 


Fig.  48. 


94 


DEMONSTRATIONS   IN    PHYSICAL   CHEMISTRY 


addition,  the  current  is  increased  to  0.22,  0.26,  0.30,  0.31, 
and  0.32  ampere  respectively,  thus  showing  that  the  in- 
crease in  strength  grows  smaller,  the  greater  the  dilution. 


Fig.  49. 

102.  Different  acids  of  the  same  molecular  concentra- 
tion exhibit  marked  differences  in  conductivity  and  hence 
in  degree  of  dissociation.  This  may  be  shown  in  a  simple 
way1  by  means  of  3  U-shaped  capillary  tubes  of  exactly 
the  same  size  (18  centimeters  long,  inner  bore  3  milli- 
meters) filled  with  normal  solutions  of  hydrochloric,  sul- 
phuric and  acetic  acid  respectively.  The  limbs  in  each 
tube  are  widened  up,  so  as  to  allow  the  passage  of  disk- 
like  platinum  electrodes  of  the  same  diameter,  placed  at 
the  same  height  in  the  solutions  (Fig.  50).  Each  tube 
is  connected  in  its  turn  with  the  aid  of  a  switch  to  a 
battery  of  20  accumulators  and  a  galvanoscope  or  milli- 

1  Riidorff-I^iipke,  1.  c.  p.  136 ;  Ostwald,  Grundlinien,  3«  Aufl.  p.  282, 
(1912). 


ELECTROCHEMISTRY   AND   IONIC   THEORY 


95 


ampere  meter.  It  will  be  seen  that  the  deviation  from  the 
zero-point  is  greatest  for  hydrochloric  acid,  somewhat 
less  for  sulphuric  acid  and  exceedingly  small  for  acetic 
acid. 


Fig.   50. 

103.  The  same  principle  can  be  demonstrated  in  a  very 
elegant  manner  with  a  more  complicated  apparatus,  de- 
vised by  Whitney  and  described  by  Noyes  and  Blanch- 
ard.1 

Four  glass  tubes,  as  nearly  alike  as  possible  (inter- 
nal diameter  3  centimeters;  length  20  centimeters),  are 
closed  at  their  lower  ends  with  a  one-holed  rubber  stop- 
per, in  which  has  been  inserted  a  thick-walled  capillary 
glass  tube  containing  a  stout  copper  wire  to  which  a  thin 
platinum  disk,  covering  the  small  end  of  the  stopper  has 
been  soldered,  and  attached  to  it  by  means  of  sealing 
wax.  The  tubes  are  set  up  in  a  vertical  position  and 
held  in  place  by  a  suitable  wooden  frame.  In  the  upper 
end  of  each  tube,  a  one-holed  rubber  stopper  is  inserted 
carrying  a  moveable  thick- walled  glass  tube  (22  centi- 
meters long)  containing  a  stout  copper  wire,  to  the  lower 
end  of  which  is  soldered  a  thin  platinum  disk  (diameter 

1 1.  c.  p.  736. 


96 


DEMONSTRATIONS    IN    PHYSICAL   CHEMISTRY 


about  2.8  centimeters)  reinforced  by  a  conical  layer  of 
sealing  wax.  Each  lower  electrode  is  connected  with  a 
32  candle-no  volt  lamp,  and  all  other  connections  made 
as  shown  in  Fig.  51.  The  upper  electrodes  are  connected 


Fig.  51. 

through  an  open  switch  with  one  terminal  and  the  lamps 
with  the  other  terminal  of  an  alternating  no-volt  cir- 
cuit. (In  case  no  alternating  current  is  available,  the  up- 
per electrodes  are  suitably  shaped  conically  in  order  to 
allow  the  gases,  evolved  during  the  electrolysis,  to  escape. 
For  the  same  reason  the  upper  rubber  stoppers  must  be 


ELECTROCHEMISTRY   AND   IONIC   THEORY  97 

provided  with  a  second  hole,  and  the  circuit  be  closed  for 
as  short  a  time  as  is  necessary).  After  placing  120  cc.  of 
distilled  water  in  the  tubes,  they  are  filled  with  5  cc.  of 
half-normal  solutions  of  hydrochloric  acid,  sulphuric 
acid,  monochlor-acetic  acid  (freshly  prepared)  and  acetic 
acid  respectively  and  the  mixtures  thoroughly  stirred. 
The  upper  electrodes  are  re-inserted  at  the  same  height, 
(one-third  of  the  distance  from  the  bottom),  the  lecture 
room  is  somewhat  darkened  and  the  circuit  closed.  The 
lamp  beneath  the  hydrochloric  acid  solution  is  found  to 
glow  brightest,  the  resistance  in  this  case  being  least; 
the  other  lamps  follow  in  brightness  in  the  order  given 
above,  the  fourth  lamp  not  glowing  perceptibly. 

The  electrodes  are  next  adjusted  so  that  the  lamps  are 
equally  bright,  when  it  is  seen,  on  re-admitting  light  to 
the  room,  that  if  the  upper  electrode  in  the  hydrochloric 
acid  is  at  the  top,  in  the  second  solution  (H2SO4)  it  is 
about  one-quarter  of  the  distance  down,  in  the  third 
(CH2C1COOH),  three-quarters  of  the  distance  down, 
while  in  the  acetic  acid  tube  both  electrodes  are  almost  in 
contact.  Finally,  in  order  to  show,  that  the  alkali  salts 
of  these  acids  all  have  nearly  the  same  conductivity  and 
degree  of  dissociation,  the  solutions  are  neutralized 
(about  the  same  amount  of  potassium  hydroxide  being 
required  in  each  case)  and  then  the  equal  brilliancy  re- 
established. It  will  be  found  this  time  that  the  upper 
electrodes  stand  approximately  at  the  same  height.  The 
same  apparatus  may  be  used  for  the  demonstration  of  the 
so-called  Ostwald's  dilution  law  and  for  the  illustration 
of  the  conductivity  and  dissociation  of  other  substances.1 

1  Noyes  and  Blanchard,  1.  c.  p.  739. 


98  DEMONSTRATIONS    IN    PHYSICAL   CHEMISTRY 

104.  That  the  "strength"  of  acids  does  not  bear  any 
relation  to  the  "potential"  amount  of  hydrogen  ions,  as 
found  by  titration  (see  foregoing  experiment),  but  is  in- 
timately connected  with  the  "actual"  amount  of  H'-ions 
in  solution,  may  be  further  illustrated  by  the  different 
speed  of  reaction  of  normal  solutions  of  different  acids  on 
equal-sized  pieces  of  metal   (zinc  or  magnesium).     On 
bringing  the  dilute  acids 

(N  HC1,  N  H2SO4,  N  CH3COOH) 
with  the  metal  in  small  Erlenmeyer  flasks,  connected 
through  rubber  tubing  with  gas  collecting  tubes  of  the 
same  size  and  diameter,  the  volumes  of  gas,  collected 
over  water  in  the  same  time  (5-10  minutes),  are  dif- 
ferent. The  solutions  are  most  suitably  treated  before- 
hand with  equal  amounts  of  a  dilute  copper  sulphate  so- 
lution and  the  gases  allowed  to  escape  for  some  time,  be- 
fore the  experiment  is  started.1 

V.   The  Common  Ion  Effect. 

The  effect  of  a  common  ion  represents  a  special  case  of 
the  mass  action  principle,  of  which  several  instances  were 
given  in  Chapter  V,  some  other  examples  will  be  discussed 
in  the  chapter  on  solubility.  Some  further  applications, 
in  which  the  dissociation  of  either  H*-ions  or  OH' -ions 
is  driven  back  by  the  addition  of  salts  with  common 
anions  or  cations,  may  be  considered  here. 

105.  H'-ions.     A  typical  case  is  the  following,  given 
by  Crum  Brown.2    A  dilute  solution  of  ferrous  sulphate 

1  Ostwald,  Grundlinien,  p.  281 ;  Rudorff-I<upke,  Grundrisz,  p.  136. 

2  Proc.  Royal  Soc.  of  Edinburgh,  21,  p.  57,  (1896). 


ELECTROCHEMISTRY   AND    IONIC    THEORY  99 

or  ferrous  ammonium  sulphate  (Mohr's  salt)  is  acidified 
with  such  an  amount  of  acetic  acid  solution,  that  on  ad- 
dition of  hydrogen  sulphide  gas  no  ferrous  sulphide  is 
precipitated.  On  throwing  a  few  particles  of  solid  so- 
dium acetate  in  the  solution,  each  crystal  forms  the  start- 
ing point  of  a  long  streak  of  iron  sulphide.  The  experi- 
ment fits  admirably  well  for  projection  on  the  screen. 

106.  The  same  result  is  obtained  by  the  use  of  an  indi- 
cator,1 viz.  methyl  orange,  which  colors  a  dilute  acetic 
acid  solution  pink.  On  adding  a  large  volume  (about  six  to 
eight  times  the  amount  of  acid)  of  a  10  per  cent,  solution 
of  sodium  acetate,  which,  to  prevent  the  suspicion  of  alka- 
linity, has  been  made  very  slightly  acid,  the  color  turns 
to  a  bright  yellow.     If,  instead  of  methyl  orange,  congo 
red  is  used  as  an  indicator,  the  observed  change  in  color 
is  from  blue  to  violet  red. 

107.  Ostwald2  has  shown  the  effect  of  salt  addition  on 
acids  with  common  anion  in  the  following  manner : 

In  two  small  conical  flasks  are  placed  two  equal-sized 
pieces  of  zinc,  covered  by  dilute  acetic  acid.  The  escaping 
gases  are  collected  after  some  time  over  water  or  glycer- 
ine in  eudiometer  tubes.  The  distance  between  the  gas 
bubbles  in  both  tubes  is  observed.  A  concentrated  so- 
dium acetate  solution  is  poured  into  the  flask,  from  which 
the  bubbles  are  coming  forth  somewhat  faster  than  in 
the  other.  The  gas  evolution  instantly  slackens  and  con- 
sequently the  distance  between  the  rising  gas  bubbles  is 
considerably  increased. 

1  Kiister,  1.  c.  p.  109. 

2  Ostwald,  Wiss.  Grundlagen  der  Anal.  Chem.,  5«    Aufl.,  p.  225,  (1910). 


IOO         DEMONSTRATIONS   IN    PHYSICAL   CHEMISTRY 

108.  Another   good    illustration    is    given   by   Tread- 
well.1     On  adding  dilute  acetic  acid  to  a  solution  con- 
taining potassium  iodide  and  potassium  nitrite,  the  solu- 
tion turns  yellow  or  brown,  owing  to  the  separation  of 
iodine,  in  accordance  with  the  equation: 

2HI  +  2HNO2  ==  2H2O  -f  2NO  -f  Ir 
If,  however,  before  adding  the  acid,  the  solution  is 
mixed  with  a  concentrated  potassium  (or  sodium)  ace- 
tate solution,  the  addition  of  dilute  acetic  acid  causes  no 
separation  of  iodine,  although  enough  H'-ions  are  present 
in  solution  to  turn  blue  litmus  paper  red.  This  proves 
that  the  amount  of  H'-ions  is  not  sufficient  to  reduce  the 
nitrous  acid.  Addition  of  a  few  drops  of  a  strong  min- 
eral acid  causes  the  immediate  liberation  of  iodine. 

109.  OH'-ions.    The  forcing  back  of  the  concentration 
of  hydroxyl-ions  is  easily  carried  out  as  follows :    A  con- 
centrated   ammonium    chloride    solution    is    made    very 
slightly  alkaline  with  ammonia,  so  as  to  make  sure  that 
it  contains  no  free  acid  (hydrolysis  is  likely  to  cause  a 
slight  acidity).     It  is  then  poured  into  an  aqueous  solu- 
tion of  ammonia,  colored  by  phenolphthalein.     The  red 
color  fades  to  a  scarcely  perceptible  pink. 

VI.   Hydrolysis. 

The  slight  ionization  of  water  (according  to  Kohl- 
rausch  and  Heydweiller2  the  degree  of  dissociation  for 
H'  and  OH'-ions,  expressed  in  gram  equivalents  per  liter 
is  1.05  times  io~7  at  25°)  accounts  for  the  hydrolysis  of 

1  Analytische  Chemie,  7e  Aufl.,  p.  311,  (1911). 

2  Zeitschr.  f.  phys.  Chem.,  14,  p.  317,  (1894). 


ELECTROCHEMISTRY   AND    IONIC   THEORY  IOI 

salts,  viz.,  the  decomposing  effect  of  water  on  dissolved 
salts.  The  salts  of  a  strong  base  and  a  strong  acid  are 
not  appreciably  hydrolyzed;  the  solutions  show  a  neutral 
reaction.  However,  under  exceptional  conditions,  water 
is  able  to  decompose  for  example  sodium  chloride,  ac- 
cording to  the  equation : 

NaCl  +  HOH  =:=  NaOH  +  HC1. 

110.  The  following  experiment  demonstrates  this  de- 
composition.1 

A  platinum  crucible  is  heated  in  a  blast  lamp  or  over 
a  Meker  burner  to  a  bright  yellow  heat  (1100°)  and  a 
small  quantity  of  sodium  chloride  (melting  point  884°) 
fused  in  the  crucible.  One  cubic  centimeter  of  water  is 
dropped  from  a  pipette  drop  by  drop  on  the  fused  salt. 
When  half  of  the  water  has  been  evaporated,  which  takes 
about  30  seconds,  the  remaining  aqueous  solution  is 
poured  off  into  a  beaker  containing  a  blue  litmus  solution 
which  turns  red.  After  the  crucible  has  cooled  down  the 
residual  salt  is  dissolved  in  water,  and  the  solution  poured 
into  a  solution  of  red  litmus  which  changes  into  blue. 

111.  Salts  of  weak  acids  with  strong  bases,  show  an 
alkaline  reaction,  readily  detected  by  litmus,  viz:    Aque- 
ous solutions  of  alkali  salts  of  hydrocyanic  acid,  carbonic 
acid,   boric  acid,  hydrosulphuric  acid.     Addition  of  a 
little  alkali  hydroxide  will   stop  hydrolysis   as   can   be 
proved  in  the  case  of  potassium  cyanide  solution  when 
the  characteristic  odor  of  hydrocyanic  acid  disappears. 

112.  Salts  of  strong  acids  with  weak  bases  react  acidic 
as  for  instance  solutions  of  copper  sulphate,  zinc  sul- 

1  Stieglitz,  1.  c.  p.  179. 


IO2         DEMONSTRATIONS   IN    PHYSICAL   CHEMISTRY 

phate,  ferric  and  aluminium  chloride,  bismuth  and  anti- 
mony chloride.  Addition  of  acid  in  this  case  prevents 
or  at  least  forces  back  hydrolysis  and  is  always  applied 
in  case  oxy-salts  should  be  formed.  Thus  the  brown- 
ish red  color  of  acqueous  ferric  sulphate  disappears  on 
the  addition  of  sulphuric  acid. 

113.  Salts  of  weak  acids  and  weak  bases  are  largely 
hydrolyzed  and  react  either  alkaline  or  acidic,  according 
to  the  nature  of  the  base  and  acid.     In  case  both  are 
equally  weak,  the  solution  is  neutral  :  ammonium  acetate 
for  instance.    If  the  acid  is  stronger  the  solution  is  acidic, 
if  the  base  is  stronger  the  solution  is  basic.     Thus  am- 
monium carbonate  reacts  alkaline,  while  ferric  acetate  is 
acid. 

Hydrolysis  is  favored  both  by  dilution  and  by  boiling. 

114.  The  effect  of  dilution  may  be  shown  by  adding 
water  to  a  slightly  acid  solution  of  bismuth  chloride  : 

BiCl3  +  H2O  ^  BiOCl  +  2HC1. 

115.  The  effect  of  boiling  is  demonstrated  by  heating 
a  ferric  acetate  solution  prepared  by  treating  freshly  pre- 
cipitated ferric  hydroxide  with  dilute  acetic  acid  when  a 
voluminous  precipitate  of  basic   ferric  acetate  will   be 
formed  : 


Fe(C,H,0,)3  +  2HJ0  =  F<  +  2HC2HSO,. 


That  boiling  favors  hydrolysis  is  due  to  the  fact  that 
the  water  is  dissociated  to  a  greater  extent  at  its  boiling 
temperature,  than  when  cold.  From  the  data  obtained  by 
Kohlrausch  and  Heydweiller  (1.  c.)  it  will  be  seen  that 


ELECTROCHEMISTRY   AND   IONIC   THEORY  IO3 

the  dissociation  at  50°  is  three  times  greater  than  at  or- 
dinary temperature  and  at  the  boiling  point  about  ten 
times  greater. 

116.  Schoorl1  recommends  the  change  in  color  of  aque- 
ous indicator  solutions  on  boiling  as  an  easy  way  for 
demonstrating  the  increasing  dissociation  of  water  with 
the  rise  of  temperature. 

To  600  cc.  of  pure,  distilled  water  is  added  3-5  cc. 
of  the  indicator  solution,  and  then  a  few  drops  of  a  base 
or  an  acid,  in  order  to  bring  the  color  to  its  neutral  point 
("mix-color").  The  solution  is  divided  into  three  equal 
parts  and  each  portion  is  poured  into  a  500  cc.  flat- 
bottom  flask  of  Jena  or  Pyrex  glass,  previously  steamed 
out.  Two  of  the  solutions  are  made  slightly  alkaline  and 
acid  respectively  and  kept  for  comparison.  The  third 
flask,  closed  by  a  plug  of  cotton  wool  is  heated  on  a  wire 
gauze.  A  change  in  color  is  visible,  even  before  the  boil- 
ing point  is  reached.  By  placing  the  flask  in  cold  water, 
the  original  color  returns.  Indicators  sensitive  to  bases, 
cause  a  change  in  color  towards  the  alkaline  color.  Thus 
methyl  orange  turns  yellow,  congo  red  changes  from  vio- 
let to  red,  lacmoid  from  violet  to  blue.  Semi-sensitive  or 
neutral  indicators  (litmus),  remain  unchanged,  while  in- 
dicators, sensitive  to  acids  show  a  change  in  color 
towards  the  acid  color,  e.  g.,  phenolphthalein  turns  from 
pink  to  colorless. 

The  following  table  gives  the  approximate  concentra- 
tion of  H'-  and  OH'-ions  at  25°  expressed  in  gram-equiva- 

i  Chem.  Weekblad,  3,  p.  771,  (1906). 
8 


104         DEMONSTRATIONS    IN    PHYSICAL   CHEMISTRY 


lents  per  liter,  at  which  a  marked  change  in  color  is  per- 
ceptible. 


Indicator 

H-  i  X  to"  3 
OH'  I  X  io—  IX 

H'  i  X  IQ—  4 
OH'  i  X  io—  I0 

H-  i  X  io—5 
OH'  i  X  iQ—9 

H-  i  X  lo"6 
OH'  i  X  io~8 

Methyl  orange 
Congo  red  •  •  . 

LitmUS  

red  orange 
blue 

orange 
violet 

yellow 
red 

red 

Phenol- 
phthalein 

Indicator 

H-  i  x  io—  7 
OH'  i  X  iQ—7 

H'  i  X  io—  8 
OH'  i  X  io—  6 

H'  i  X  io-9 
OH'  i  X  io~5 

H'  i  X  io—  I0 
OH'  i  X  io—4 

Methyl  orange 
Congo  red  -  .  . 

hlup 

Phenol- 
phthalein 

colorless 

pink 

red 

VTI.   lonization  and  Chemical  Activity. 

117.  lonization  easily  explains  the  immediate  interac- 
tion of  chemical  compounds  in  aqueous  solution,  and  the 
inactivity  or  slow  reaction  in  non-aqueous  (non-ionized) 
solutions.  A  comparison  of  the  properties  of  hydro- 
chloric acid  dissolved  in  water  and  in  toluene  may  be 
taken  as  an  illustration. 

An  aqueous  solution  of  hydrochloric  acid  dissolves 
marble  rapidly  while  a  solution  in  toluene,  a  non-con- 


ELECTROCHEMISTRY   AND   IONIC   THEORY  10$ 

ductor  of  electricity,  has  little  or  no  effect  on  calcium 
carbonate. 

118.  Another  experiment  of  the  same  kind  is  the  fol- 
lowing:1    One  hundred  cc.  of  nearly  saturated  aqueous 
solutions  of  potassium  bromide  and  zinc  bromide  and 
equal  volumes  of  5  per  cent,  alcoholic  solutions  of  isopro- 
pyl  bromide,  ethyl  bromide  and  phenyl  bromide  are  pre- 
pared and  placed  in  five  lecture  jars.    A  nearly  saturated 
alcoholic  solution  of  silver  nitrate  (2.5  grams  in  100  cc.) 
is  then  prepared  and  20  cc.  of  this  solution  added  to  each 
of  the  five  jars  after  which  the  mixtures  are  thoroughly 
stirred.     An   immediate   precipitate    is    formed    in   the 
first  two  cases  in  which  the  reactions  are  ionic.    An  in- 
complete reaction,  although  immediate  turbidity  is  visible, 
is  shown  in  the  case  of  isopropyl  bromide.    A  slow  reac- 
tion takes  place  in  the  ethyl  bromide  solution,  while  ap- 
parently no  reaction  is  going  on  in  the  solution  containing 
phenyl  bromide.     Care  must  be  taken  that  the  organic 
bromides  are  free  from  hydrobromic  acid  and  bromine, 
from  which  they  may  be  freed  by  washing  first  with  a 
dilute  sodium  carbonate  solution  and  then  with  water. 

119.  As  has  been  pointed  out  by  Kahlenberg,2  this  dif- 
ference in  behavior  need  not  necessarily  be  a  consequence 
of  electrolytic  dissociation.     In  fact,  it  has  been  proved 
that  copper  oleate  dissolved  in  benzene  or  toluene  gives 
an  immediate  precipitate  of  copper  chloride  with  hydro- 
chloric acid,  phosphorous  trichloride,  tin  tetrachloride, 
antimony  trichloride,  etc.,  dissolved  in  these  same  hydro- 

1  Noyes  and  Blanchard,  1.  c.  p.  728. 

2  Outlines  of  Chemistry,  revised  ed.,   New  York,  p.  453,  (1916) ;  Journ.  of 
phys.  Chem.  6,  p.  i,  (1902). 


IO6         DEMONSTRATIONS    IN    PHYSICAL   CHEMISTRY 

carbons,  notwithstanding  the  fact  that  all  these  solutions 
are  non-electrolytes.  In  the  same  way,  as  may  be  shown 
in  the  lecture,  sodium  alcoholate  and  ferric  chloride  (su- 
blimed) both  dissolved  in  absolute  ethyl  alcohol,  react  with 
the  formation  of  ferric,  alcoholate  and  the  direct  precip- 
itation of  sodium  chloride.  Recent  investigations  by  Cady 
and  Lichtenwalter,1  however,  lead  to  the  conclusion  that 
these  "exceptional"  cases  are  in  agreement  with  the  dis- 
sociation theory. 

VIII.   lonization  and  Color  of  Solutions. 

120.  All  salts  of  a  metal,  whose  ion  is  colored,  show  the 
same  color  in  dilute  aqueous  solutions,  independent  of  the 
anion,  present  in  the  solution  (provided  the  latter  is  col- 
orless), whatever  the  colors  of  the  undissociated  salts 
may  be. 

Thus  dilute  aqueous  solutions  of  copper  sulphate, 
acetate,  chloride,  and  nitrate  all  show  the  same  blue  color 
due  to  the  Cu"-ion.  On  the  other  hand,  the  same  salt, 
when  dissolved  in  a  small  amount  of  water  or  dissolved  in 
solvents  other  than  water,  very  often  shows  a  different 
color : 

Twenty-seven  grams  of  anhydrous  copper  chlor- 
ide, dissolved  in  100  cc.  of  absolute  alcohol  gives  a  dark 
green  solution ;  about  the  same  color  is  exhibited  by  a  con- 
centrated aqueous  solution.  On  diluting  with  water,  both 
show  the  same  shade  of  blue. 

121.  Characteristic  changes  in  color  occur  with  cobalt 
solutions.    Alcoholic  solutions  of  cobalt  chloride  and  ni- 

1  Journ.  Am.  Chem.  Soc.,  35,  p.  1434,  (1913). 


ELECTROCHEMISTRY   AND    IONIC    THEORY  IO/ 

trate  are  deep  violet-blue  and  purplish-red  respectively. 
The  solutions  are  made  up1  by  dissolving  60  grams  of 
CoCL2.6H2O  and  73  grams  of  Co(NO3)2.6H2O  in  100  cc. 
of  ethyl  alcohol  (96  per  cent.).  On  adding  water  to  the 
solutions,  they  both  turn  pink.  The  reverse  change :  pink 
•-*  blue  is  observed  when  a  concentrated  hydrochloric 
acid  solution  is  added  to  a  pink-colored  aqueous  solution 
of  cobalt  chloride. 

The  original  idea  of  Ostwald,  that  the  change  in  color 
in  this  case  is  solely  due  to  a  change  in  dissociation,  ac- 
cording to  the  equation : 

CoCl,  ^  Co"  -f  2C1', 

must  be  abandoned  in  the  light  of  recent  investigations.2 
It  is  highly  probable  that  in  this  and  similar  other  cases 
the  change  in  color  is  mainly  caused  by  hydratation  of  the 
salts,  dissociation  being  of  minor  importance. 

1  Noyes  and  Blanchard,  1.  c.  p.  727. 

2  cf.  Bottger,  Qualitative  Analyse,   3«  Aufl.   p.  223,  (1913).     Hantzsch,  Zeit- 
schr.  f.  anorg.  Chem.,  73,  p.  309,  (1912). 


CHAPTER  VIII. 


SOLUBILITY  AND  ITS  CHANGES, 

In  this  chapter,  which  needs  no  special  description  of 
apparatus,  most  experiments  being  conveniently  carried 
out  in  large  test-tubes,  only  a  few  suitable  combinations 
of  liquids  and  solids  are  given,  in  order  to  illustrate  the 
more  common  cases  of  solubility.  These  may  be  divided 
in  three  groups,  demonstrating : 

I.  Solubility  of  liquids. 
II.  Solubility  of  one  solute  in  two  solvents. 
III.  Solubility  of  salts  in  water. 

I.   Solubility  of  Liquids. 

122.  A  case  of  miscibility  in  all  proportions  is  that  of 
alcohol  and  water  (consolute  liquids). 

123.  Partially  miscible  are  water  and  butyl   alcohol. 
Equal  volumes  of  both  liquids  are  shaken  in  a  test-tube 
and   equilibrium  established  at  25°.     On  inserting  the 
tube  in  a  beaker  of  hot  water  (about  40°)  without  any 
further  shaking,  the  lower  layer,  being  an  aqueous  solu- 
tion of  butyl  alcohol,  becomes  turbid.     On  putting  the 
tube  in  a  beaker  of  cold  water  (about  10°)  the  reverse 
happens:  the  lower  layer  clears  up  and  the  supernatant 
liquid  turns  milky  by  the  separation  of  water  drops.1 

124.  A  mixture  of  36  per  cent,  phenol  and  64  per  cent, 
water  forms  a  two-layer  system,  but  on  raising  the  tem- 
perature the  mutual  solubility  increases,  so  that  at  68.8,° 

1  Miiller  and  Abegg,  Zeitschr.  f.  Electrochemie,  n,  p.  3,  (1905). 


SOLUBILITY   AND   ITS   CHANGES  IOO, 

where  the  "upper"   critical  temperature  of   solution  is 
reached,  a  homogeneous  liquid  forms.1 

125.  A  "lower"  critical  temperature  of  solution  is  ex- 
hibited at  1 8.6°,  by  a  mixture  of  52  per  cent  tri-ethyl 
amine  and  48  per  cent,  water.2    On  account  of  the  sharp 
odor  of  the  amine  the  mixture  should  be  kept  in  a  sealed 
tube.     Newth3  recommends  this  couple  as  an  extremely 
sharp  indicator  for  slight  changes  in  temperature,  a  low- 
ering of  the  temperature  below  18.6°  being  immediately 
shown  by  a  milky  appearance,  followed  by  the  formation 
of  two  layers  from  the  homogeneous  liquid. 

126.  Nicotine  and  water  is  a  most  interesting  combina- 
tion because  it  is  characterized  by  the  occurrence  of  an 
upper  and  lower  critical  temperature  of  solubility  at  210° 
and  61°  respectively  for  a  mixture  of  32  per  cent,  nicotine 
and  68  per  cent,  water.*    This  may  be  shown  by  heating 
the  mixture  in  a  small  strong  well  sealed  glass  tube  (in- 
ner bore  0.5  centimeter,  length  10  centimeters)   taking 
proper  precautions  in  case  the  tube  should  crack.       If 
the  experiment  is  to  be  shown  to  a  large  audience,  a 
projection  apparatus  may  be  used. 

II.  Solubility  of  One  Solute  in  Two  Solvents. 

127.  The  relative  decrease  in  solubility  of  one  solvent 
as  compared  with  a  second  by  the  addition  of  a  solid, 
soluble  in  both  solvents,  may  be   demonstrated  by  the 
couple  water  and  ethyl  ether  on  adding  naphthalene.    For 

1  Rothmund,  Zeitschr.  f.  phys.  Chem.,  26,  p.  452,  (1898). 

2  Rothmund,  Ibidem,  p  459. 

s  Chemical  lecture  experiments,  Condon,  p.  69,  (1899). 
*  Hudson,  Zeitschr.  f.  phys.  Chem.,  47,  p.  113,  (1904). 


IIO         DEMONSTRATIONS   IN    PHYSICAL   CHEMISTRY 


this  experiment  a  flask  with  a  long  calibrated  bulb- 
neck,  as  devised  by  Tolloczko,1  is  used  (Fig.  52).  The 
flask  is  filled  to  the  mark  (a)  with  water 
saturated  with  ether,  and  then  enough  ether 
saturated  with  water  is  added  to  fill  the 
neck  up  to  the  mark  (  b ) .  In  order  to  get  a 
visible  boundary  line,  the  ether  is  slightly 
colored  with  a  dye,  insoluble  in  water.  On 
pouring  successively  equal  weights  (0.25 
gram)  of  naphthalene  into  the  flask,  it  will 
be  found,  that  each  time — after  vigorous 
shaking — ,  the  boundary  is  displaced  down- 
ward for  about  the  same  number  of  scale 
divisions,  showing  that  the  addition  of 
naphthalene  causes  a  decrease  of  the  solu- 
bility of  ether  in  water  proportional  to  the 
Fig.  52.  weight  of  the  dissolved  solid. 

128.  The  solubility  of  a  solid  may  be  decreased  by  the 
addition  of  another  solvent  which  is  consolute  with  the 
first,  like  water  and  ethyl  alcohol.     On  adding  alcohol  to 
a  saturated  solution  of  cane  sugar  in  water,  part  of  the 
sugar  is  precipitated. 

129.  A  homogeneous  mixture  of  two,  wholly  or  parti- 
ally, miscible  liquids  may  be  separated  into  two  layers 
by  the  addition  of  a  salt.     Thus,  on  adding  some  solid 
potassium  sulphate  to  an  aqueous  solution  of  phenol,  the 
latter  separates  in  the  form  of  a  milky  emulsion  which 
changes  into  a  liquid  layer  after  a  while.     This  is  the 
process,   well-known   in   organic   chemistry,   of   "salting 

i  Zeitschr.  f.  phys.  Chetn.,  20,  p.  389,  (1896). 


SOUJBIUTY   AND    ITS   CHANGES  III 

out."  It  is  by  this  method,  that  compounds  like  alcohol, 
aceton,  etc.,  which  are  very  soluble  in  water,  are  easily 
separated  out. 

130.  As  an  exception  to  the  rule  that  the  solubility  of 
a  non-electrolyte  in  water,  diminishes  by  the  addition  of  a 
salt,  the  case  of  iodine  and  water  may  be  noted.     To  a 
saturated  solution  of  iodine  in  water,  with  an  excess  of 
iodine  at  the  bottom  of  the  test-tube,  some  crystals  of 
potassium  iodide  are  added.     On  vigorously  shaking  the 
mixture,  the  iodine  goes  in  solution,  forming  as  is  well 
known  a  dark  brown  solution  with  the  salt.     The  in- 
crease in  solubility  is  accounted  for  by  the  formation  of 
complex  ions.1 

m.   Solubility  of  Salts  in  Water. 

131.  The  solubility  of  potassium  nitrate  increases  very 
rapidly  as  the  temperature  rises,  while  that  of  sodium 
chloride   is   nearly   independent  of   the  temperature;   a 
decrease  of  solubility  on  raising  the  temperature  occurs 
in  the  case  of  lithium  sulphate.     The  heat  absorption 
and  the  resulting  fall  in  temperature  on  dissolving  the 
potassium  nitrate  and  conversely  the  rise  in  temperature, 
due  to  heat  evolution,  when  lithium  sulphate  is  dissolved 
(at  room  temperature)  may  be  demonstrated  in  addition, 
with  the  aid  of  a  large  air  or  liquid  thermometer,  as  used 
for  lecture  experiments. 

132.  A  decrease  in  the  solubility  of  salts  is  generally 
observed  on  addition  of  salts  with  common  ions. 

Thus  addition  of  concentrated  hydrochloric  acid  to  sat- 

1  I^eblanc  and  Noyes,  Zeitschr.  f.  phys.  Chem.,  6,  p.  401,  (1890). 


112         DEMONSTRATIONS   IN    PHYSICAL   CHEMISTRY 

urated  solutions  of  sodium  chloride,  potassium  chloride, 
barium  chloride,  etc.,  causes  an  immediate  precipitation 
of  the  salt  in  question.1 

In  the  same  way  lead  chloride  is  precipitated  from  its 
saturated  solution  by  the  addition  of  concentrated  sodium 
chloride  solution.2 

Nernst  was  the  first  who  showed  that  an  excess  of 
either  the  anion  or  the  cation  of  the  salt  in  solution  causes 
precipitation  from  its  saturated  solution.  He  found  that 
an  addition  of  concentrated  potassium  hydroxide  solution 
or  potassium  chloride  solution  to  10  cc.  of  a  saturated 
solution  of  potassium  chlorate  gave  in  the  former  case 
directly,  in  the  latter  after  a  few  minutes,  an  abundant 
precipitate  of  potassium  chlorate.  The  same  result  was 
obtained  by  the  addition  of  a  very  concentrated  solution 
of  sodium  chlorate.3 

133.  The  precipitation  of  silver  acetate  from  its  satur- 
ated solution  by  the  addition  of  either  silver  nitrate  or 
sodium  acetate  solution  is  another  instance,  described  in 
detail  by  A.  A.  Noyes  and  Blanchard.4 

Five  hundred  cubic  centimeters  of  a  saturated  solution 
of  silver  acetate  is  prepared  by  shaking  an  excess  of  the 
salt  with  warm  water,  cooling  and  filtering.  In  each  of 
two  lecture  jars  is  placed  200  cc.  of  this  solution  and  then 
5  cc.  of  a  4N.  solution  of  sodium  acetate  added  to  one  jar 
and  5  cc.  of  a  4N  solution  of  silver  nitrate  to  the  second. 
On  vigorously  stirring  for  a  few  moments,  in  both  jars  a 
feathery,  crystalline  precipitate  is  produced. 

1  Engel,  Ann.  de  chim.  et  phys.,  (6)  13,  p.  132,  (1888). 
2Kiister  I.e.  p.  in. 

3  Nernst,  Zeitschr.  f.  phys.  Chem.,  4,  p.  372,  (1887). 
M.  c.  p.  750. 


SOUJBIUTY  AND   ITS   CHANGES  113 

134.  However  convincing  these  experiments  may  be  as 
viewed  from  the  theory  given  by  Nernst,  more  recent  in- 
vestigations by  A.  A.  Noyes  and  his  pupils  have  proved 
that  the   principle   of   the  constancy   of   the  solubility- 
product  and  of  the  concentration  of  the  non-ionized  salt 
in  the  saturated  solution  as  derived  from  the  law  of 
mass-action,  cannot  be  universally  applied.     To  cite  one 
example  :     the  solubility  of  thallous  sulphate  (T12SO4)  is 
increased  by  the  addition  of  solutions  of  sodium  sulphate, 
sodium  hydrosulphate  and  even  more  of  sulphuric  acid.1 
Some  exceptions  find  a  suitable  explanation  in  the  as- 
sumption  of   the   formation   of   complex   ions,   e.  g.   by 
bringing  together  solutions  of  potassium  nitrate  and  lead 
nitrate. 

135.  An  increase  in  solubility  of  salts,  hydroxides  and 
oxides,  that  are  slightly  soluble,  is  usually  observed,  in 
accordance  with  a  rule,  formulated  by  A.  A.  Noyes,2  on 
the  addition  of  salts  without  a  common  ion,  and  especi- 
ally when  there  is  an  opportunity  for  forming  non-  (or 
slightly)  ionized  salts  by  double  decomposition. 

Thus  mercuric  oxide,  which  is  hardly  soluble  in  water, 
goes  into  solution  by  shaking  vigorously  with  an  alkali 
chloride  solution  and,  still  better,  on  applying  a  solution 
of  potassium  cyanide,  mercuric  cyanide  being  scarcely 
ionized.3 

The  case,  well-known  in  analytical  chemistry,  of  mag- 
nesium hydroxide,  dissolving  on  the  addition  of  ammon- 
ium salts  is  another  example  belonging  to  this  class.4 


1  Harkins,  Tourn.  Am.  Chem.  Soc.,  33,  p.  1836,  (1911). 

2  Zeitschr.  f.  phys.  Chem.,  6,  p.  262,  (1890). 
*  Bersch,  Ibidem,  8.  p.  383,  (1891). 

4  I^oven,  Zeitschr.  f.  anorg.  Chem.,  11,  p.  404,  (1896). 


CHAPTER  IX. 


COLLOIDS  AND  ADSORPTION. 

The  importance  of  colloid  chemistry  need  not  be  em- 
phasized, since  it  has  been  recognized  that  in  nature 
crystalloid  behavior  is  the  exception,  colloid  behavior  the 
rule.  Since  the  time  when  Thomas  Graham,  the  father 
of  colloid  chemistry,  made  his  famous  discovery  which 
led  to  the  distinction  of  crystalloids  and  colloids1  (1861), 
this  subject  has  been  invariably  dealt  with  in  almost  every 
textbook  of  inorganic  chemistry  in  the  chapter  on  silicon, 
as  if  the  colloid  state  was  primarily  characteristic  of  this 
substance.  During  the  past  20  years,  however,  it  has 
been  realized  that  colloid  properties  are  not  connected 
with  some  definite  chemical  composition,  but  that  all  ma- 
terials may  occur  in  the  colloid  state.  Chemistry  is  largely 
indebted  to  Wolfgang  Ostwald,  the  editor  of  the  Kolloid- 
Zeitschrift,  for  the  assumption  of  the  universality  of  the 
colloid  state  and  the  recognition  of  the  essential  independ- 
ence of  the  latter  in  regard  to  chemical  composition.  The 
chief  characteristic  of  a  colloid  substance  lies  in  its 
specific  surface  (expressed  by  the  quotient  of  absolute 
surface  by  the  total  volume  of  the  disperse  phase,  accord- 
ing to  W°  Ostwald).  It  is  therefore,  readily  understood, 
that  ad-(ab)  sorption  is  intimately  connected  with  colloids 
and  for  this  reason  is  included  in  this  chapter.  Speaking 
of  colloids  in  general,  colloid  solutions  are  usually  meant, 
as  distinguished  from  colloid  precipitates  or  gels.  In 
the  following  experiments  colloid  solutions  only  will  be 

i  Phil.  Trans.  151,  p.  184,  (1861). 


COLLOIDS   AND   ADSORPTION  115 

considered,  and  from  the  latter  only  colloids,  in  which 
water  is  the  dispersion  medium,  while  the  finely  sub- 
divided discontinuous  phase, — the  disperse  phase — is 
either  a  solid  or  a  liquid  (perhaps  of  complex  nature). 
The  whole  system  is  conveniently  called :  dispersoid. 

This    leads    to    the    following    rough    classification: 
(W°  Ostwald.)1 

Dispersion  medium  (WATER) 


V 
Disperse  phase  :  SOLID 


I 
V 

1  °— MOLECULAR 

DISPERSOIDS 

(crystalloids.  Graham,  > 
size  about  i  /u/u.  or  less 
specific  surface  >io7.  ; 

2  ° — SUSPENSOIDS 

(colloid  solutions) 


A      A 
'  N    '  > 


V 
Disperse  phase  :   LIQUID 

(  or  a  complex 
phase=mixture  of 
mutually  soluble 
compounds).  W°. 
Ostwald. 

V 

—  MOLECULAR 

DISPERSOIDS 

(consolute  liquids) 


2 — EMULSOIDS 

(colloid  solutions) 


3°  —  SUSPENSIONS 


(true  dispersions,  size  of\     .S  4; 

particles  >O.I/IA,  specific  I     _O  j-i 

surface  <6.io&                   ^     3  a? 

O  Q 


3°  —  EMULSIONS 

(true  dispersions) 


As  will  be  seen  from  this  table  colloid  solutions  (sus- 
pensoids  and  emulsoids)  occupy  an  intermediate  position, 

1  Handbook  of  Colloid  Chemistry,  Americ.  ed.,  by  M.  Fischer  (1915),  p.  33. 


Il6         DEMONSTRATIONS   IN    PHYSICAL   CHEMISTRY 

although  in  reality  no  sharp  boundary  line  can  be  drawn 
between  colloid  solutions  and  "real"  solutions  on  one 
side  or  between  colloid  solutions  and  suspensions  or  emul- 
sions on  the  other  side.  There  is,  however,  a  rather  wide 
gap  between  typical  emulsoids  and  typical  suspensoids, 
although  it  must  be  admitted  that  transitions  between 
these  two  groups  have  been  observed  in  a  number  of 
cases.1  It  has  been  found  for  instance  that  some  chemi- 
cal substances  like  soaps,  many  dyes,  etc.,  form  emuls- 
oids in  water  and  suspensoids  (or  molecularly  dispersed 
solutions)  in  alcohol.  Likewise  some  hydroxides,  especi- 
ally those  of  iron  behave  like  suspensoids  in  dilute  aqueous 
solutions  and  like  emulsoids  in  concentrated  solutions. 

The  experiments,  described  below,  are  subdivided  for 
the  sake  of  convenience  into  the  following  six  groups : 

A.  Preparation  of  suspensoids. 

B.  Preparation  of  emulsoids. 

C.  Mechanical  properties  of  dispersoids. 

D.  Optical  properties  of  dispersoids. 

E.  Electrical  properties  of  dispersoids. 

F.  Adsorption. 

A.  Preparation  of  Suspensoids. 

The  methods  for  preparing  suspensoids  fall  in  two 
classes :  electrical  and  (physico-)  chemical. 

136.  The  electrical  method  of  direct  disintegration 
was  first  introduced  by  G.  Bredig2  and  later  successfully 
applied  with  a  more  elaborate  arrangement  by  Th. 
Svedberg.3 

1  Ostwald-Fischer,  1.  c.  pp.  44,  45,  55,  56,  136,  147. 

2  Zeitschr  f.  phys.  Chem.,  31,  p.  258,  (1899). 

3  Ber.  d.  chetn.  Ges.,  38,  p.  3616,  (1905)  ;  39,  p.  1705,  (1906). 


COLLOIDS   AND   ADSORPTION 


117 


Following  Bredig's  directions,  two  short  wires  of 
the  metal  to  be  dispersed  (usually  platinum,  gold 
or  silver),  1-3  millimeters  in  cross-section,  are  attached 
to  stout  copper  wires.  Each  wire  is  insulated  by  slipping 
small  capillary  glass  tubes  over  it,  leaving  a  free  end  of 
i  centimeter.  Both  are  connected  with  the  terminals  of 
a  no- volt  direct  lighting  circuit,  having  a  suitable  re- 
sistance (a  20  ohm  rheostat  or  a  32  candle-power  lamp) 
in  series,  in  order  to  secure  a  current  of  4-5  ampere. 


Fig.  53. 

Pure  distilled  water  (to  which  a  trace  of  hydrochloric 
acid  may  be  added)  is  then  placed  in  a  crystallizing  dish, 
10  centimeters  in  diameter,  cooled  by  ice  water  in  a  larger 
surrounding  dish.  (Fig.  53.)  The  ends  of  both  wires 
are  then  dipped  in  the  water,  brought  in  contact  and  im- 
mediately separated  1-2  millimeters,  so  as  to  form  an 
electric  arc  in  the  water.  One  of  the  wires  is  suitably 
fastened  to  a  clamp  stand ;  the  glass-insulated  part  of  the 
other  is  grasped  in  the  hand.  The  arc  is  maintained  for 
about  10  minutes,  taking  care,  each  time  that  the  arc  dis- 
appears, to  unite  and  separate  the  ends  of  the  wires,  or 


Il8  DEMONSTRATIONS  IN  PHYSICAL  CHEMISTRY 

in  case  of  fusing  together,  to  re-form  the  required  dis- 
tance. The  solution,  thus  obtained  is  filtered  and  kept  in 
a  stoppered  bottle.  Its  color  is  greenish-brown  for  silver, 
red  for  gold  and  black  for  platinum. 

Chemical  methods  include  reductions,  double  decom- 
positions, hydrolysis  (being  a  special  case  of  the  preced- 
ing) peptizations  and  dilutions.  Reduction-methods  are, 
like  the  disintegration-method,  chiefly  confined  to  the 
noble  metals.  As  reducing  agents  yellow  phosphorus 
(Faraday,  1857),  formaldehyde  (Zsigmondy,  1898),  hy- 
droxylamine  (Gutbier),  phenyl-hydrazine  and  other, 
mostly  organic,  reducing  compounds  have  been  used. 

The  following  preparations  of  gold-  and  silver-hydro- 
sols  are  easily  made  by  the  reduction  method.1 

137.  Gold-hydrosol.  Four  cc.  of  a  I  per  cent  solution  of 
commercial  gold  chloride  are  diluted  with  100  cc.  of  dis- 
tilled  water.     A   solution   of   2  grams   tannin    (acidum 
tannicum  purissimum)  in  100  cc.  of  distilled  water  serves 
as  a  reducing  agent.    By  mixing  three  parts  of  the  latter 
solution  with  one  part  of  the  dilute  gold  chloride  solu- 
tion a  blue  gold-hydrosol  results.    By  taking  equal  parts 
a  ruby-red   hydrosol  is   formed.       The  sols  are   fairly 
stable. 

138.  Silver-hydrosols  are  less  stable.     A  useful  prep- 
aration, however,  is  obtained  in  the  following  manner. 
To  5  cc.  of  a  i  per  cent,  solution  of  silver  nitrate  is  added, 
drop  by  drop,  a  dilute  ammonia  solution,  until  the  first 
formed  precipitate  exactly  disappears,  and  then  diluted 
with  distilled  water  to  a  100  cc.  By  mixing  equal  volumes 

1  Hatschek,  Physics  and  Chemistry  of  Colloids,  Condon,  p.  8,  (1913). 


COLLOIDS  AND  ADSORPTION  IIQ 

of  this  solution  and  the  above  mentioned  tannin  solution 
a  clear  and  transparent,  brown  silver-hydrosol  results, 
which  sometimes  shows  a  green  color  in  reflected  light. 

The  third  method  to  be  considered  is  the  process  of 
double  decomposition.  This  has  to  be  carried  out  in  the 
absence  of  electrolytes,  the  latter  having  the  tendency  to 
precipitate  the  suspension  colloids.  Therefore  only  very 
dilute  solutions  can  be  used,  so  that  the  small  quantity  of 
electrolyte,  if  formed  in  the  reaction,  will  not  do  any 
harm ;  on  the  contrary  it  is  well-known  that  traces  of  elec- 
trolytes increase  the  stability  of  suspensoids. 

For  lecture  experiments  the  following  preparations  of 
this  type  may  be  performed. 

139.  By  mixing  equal  volumes  of  N/5O  ferric  chloride 
solution    and    N/5O    potassium    ferrocyanide    a    colloid 
solution  of  prussian  blue  is  obtained,1  which  is  so  dense  in 
color,  that  it  is  only  transparent  in  thin  layers. 

140.  Two  hundred  cubic  centimeters  of  a  I  per  cent,  so- 
lution of  arsenious  oxide  (As2O3),  (prepared  by  boiling 
water,  containing  12  grams  of  the  oxide,  cooling  and  ni- 
trating the  solution)  are  mixed  with  200  cc.  of  a  satur- 
ated solution  of  hydrogen  sulphide.       A  turbid  yellow 
solution  is  formed,  which  can  be  filtered  through  a  folded 
filter.2 

141.  Two  hundred  cubic  centimeters  of  a  one-eighth 
molar  solution  of  mercuric  cyanide  and  an  equal  volume 
of  a  saturated  hydrogen  sulphide  solution  are  simultane- 

1  A.  A.  Noyes,  Journ.  Am.  Chem.  Soc.,  27,   85—104,  p.  93,  (1905). 

2  Noyes,  1.  c.  p.  93. 

9 


120         DEMONSTRATIONS    IN    PHYSICAL   CHEMISTRY 

ously  poured  in  a  beaker.     The   resulting  black   liquid 
passes  almost  completely  through  a  folded  filter.1 

142.  Two  grams  of  tartar  emetic  are  dissolved  in  100 
cc.  of  water  and  mixed  with  an  equal  volume  of  the  com- 
mon strong  ammonium  sulphide  solution  diluted  to  one- 
twenty-fifth  of  its  strength.    The  color  changes  gradually 
into  orange-yellow  by  the  formation  of  an  antimony  sul- 
phide sol.2 

143.  Hydrolysis   of   salts   affords   another   means   for 
preparing   suspensoids.     Thus   a    dilute   iron   hydroxide 
sol, — which  behaves  like  a  sttspensoid — ,  of  great  stability, 
is  obtained  by  heating  500  cc.  of  water  to  boiling  in  a 
large  Erlenmeyer  flask  or  a  800  cc.  beaker,  and  adding  to 
the  boiling  water  5  cc.  of  a  33  per  cent,  solution  of  ferric 
chloride.     A  clear  sol  of  a  beautiful  reddish-brown  color 
is  formed.3 

144.  Peptization, — a  term,  first  suggested  by  Graham 
and  meaning  an  increase  in  the  degree  of  dispersion — is 
the  reverse  process  of  coagulation.     It  may  be  shown  in 
the  preparation  of  cadmium  sulphide  sol.     The  sulphide 
is  precipitated  by  hydrogen  sulphide  from  an  ammoniacal 
solution  of  cadmium  sulphate,  the  precipitate  thoroughly 
washed  and  suspended  in  distilled  water.     Thus  far  the 
experiment  may  be  completed  before  demonstrating  the 
change  from  a  suspension  into  a  suspensoid.     By  passing 
hydrogen  sulphide  through  the  suspension,  the  latter  be- 

1  Noyes,  1.  c.  p.  93. 

2  Hatschek,  1.  c.  p.  8. 
s  Hatschek,  1.  c.  p.  8. 


COLLOIDS   AND   ADSORPTION  121 

comes  milky  and  turbid,  until  finally  it  changes  into  a 
clear,  transparent  suspensoid  of  a  beautiful  gold  color.1 

145.  The  dilution  process  consists  in  pouring  a  few 
drops  of  a  non-aqueous  solution  of  the  substance  to  be  dis- 
persed in  a  large  excess  of  water.     Thus  colloid    solu- 
tions  of    sulphur   and   selenium   of   great   stability   are 
formed  by  dissolving  pulverized  sulphur  and  selenium 
(either  the  red  amorphous  or  the  greyish  crystalline  modi- 
fication) in  a  few  cubic  centimeters  of  hydrazine  hydrate 
and  pouring  2  or  3  drops  of  the  dark  viscous  liquid  in 
several  liters  of  water.       In  this  way  an  intensely  red 
colloid  solution  of  selenium  and  a  yellowish  white  col- 
loid solution  of  sulphur  are  obtained.2 

146.  In  an  analogous  manner  suspensions  of  mastic, 
gutta  percha,  etc.,  as  were  used  by  Perrin,  Henry  and 
others  in  their  study  of  the  Brownian  movement  may  be 
obtained  by  pouring  a  few  drops  of  an  alcoholic  solution 
into  an  excess  of  water. 

B.   Preparation  of  Emulsoids. 

Examples  of  this  class  of  colloids  are  the  proteins  (like 
egg-albumen),  gelatin,  agar-agar,  starch  dextrin,  many 
gums,  silicic  acid,  stannic  acid,  many  hydroxides  in 
concentrated  solution  (iron  for  instance)  dye-stuffs  (like 
night-blue,  benzo-purpurin,  azo-blue,  etc.).  The  prep- 
aration of  these  colloids  does  not  require  any  special  de- 
scription. 

147.  As  characteristic  lecture  demonstration  types,  be- 
sides the  classical  silicic  acid  usually  gelatin  or  agar-agar 

1  Prost,  Bull,  de  1'Ac.  des  sc.  de  Brux.,  (3)  14,    p.  312,  (1887). 
-  Meyer,  Ber.  d.  chem.  Ges.  46,  p.  3089,  (1913). 


122         DEMONSTRATIONS   IN    PHYSICAL   CHEMISTRY 

solutions  (containing  1-5  grams  in  100  cc.  of  water)  are 
selected. 

C.  Mechanical  Properties  of  Dispersoids. 
Among  these  will  be  described : 

I.  Diffusion. 
II.  Dialysis. 

III.  Viscosity. 

IV.  Surface  tension. 

I.   DIFFUSION  EXPERIMENTS. 

148.  An  easy  method  for  distinguishing  between  true 
solutions  and  colloid  solutions  is  based  on  diffusion.    W° 
Ostwald1  uses  5  per  cent,  gelatin  solutions  or  2  per  cent, 
agar-agar  solutions,  which,  while  hot,  are  poured  in  test- 
tubes,  until  these  are  half-way  rilled  and  then  allowed  to 
congeal.     Care  must  be  taken  that  the  gelatin  and  agar- 
agar  are  thoroughly  washed  and  purified.  The  solid  layers 
are  covered  with  safranin  and  congo-red  solutions  re- 
spectively.    The  result  of  the  diffusion  in  both  tubes  is 
clearly  visible  after  24  hours.     The  congo-red,  being  a 
colloid  has  only  slightly  spread  into  the  jelly,  while  the 
safranin  which  forms  a  true  solution  has  passed  into  it 
for  a  considerable  distance  so  that  the  originally  sharp 
boundary  surface  is  hardly  visible. 

149.  In  a  similar  way  Noyes2  shows  this  difference  in 
speed  of  diffusion  between  crystalloid  and  colloid  solu- 
tions.   Two  cylindrical  sticks  of  agar-jelly,  5  centimeters 
in  diameter  and  15  centimeters  in  height  are  prepared  by 

1  Ostwald-Fischer,  1.  c.  p.  9. 

2  Noyes,  1.  c.  p.  90. 


COLLOIDS   AND   ADSORPTION  123 

pouring  a  hot  4  per  cent,  solution  of  agar-agar  into  large 
glass  tubes,  corked  at  one  end.  When  cold,  the  solid 
sticks  are  pushed  out  and  placed  in  two  lecture  jars  of 
which  one  is  half-way  filled  with  a  nearly  saturated  solu- 
tion of  copper  sulphate,  treated  with  enough  ammonium 
hydroxide  to  redissolve  the  precipitate  first  formed, 
while  the  other  contains  a  colloid  solution  of  prussian 
blue,  made  by  mixing  equal  volumes  of  N/5O  solutions 
of  ferric  chloride  and  potassium  ferrocyanide.  After 
the  diffusion  has  been  in  progress  for  two  days,  the  re- 
sult is  shown  to  the  audience  by  removing  the  sticks  and 
cutting  them  in  two.  The  blue  copper  solution  has  pene- 
trated the  stick  uniformly  to  the  very  center,  while  the 
prussian  blue  has  not  entered  into  the  stick  over  any 
perceptible  distance,  thus  proving,  that  colloid  solu- 
tions practically  do  not  diffuse  at  all.  Accurate  measure- 
ments by  Voigtlander1  have  brought  out  that  the  diffusion 
of  crystalloids  is  not  appreciably  influenced  by  jellies, 
provided  these  are  not  present  in  greater  percentage  than 
3-5  per  cent. 

II.   DIALYSIS  EXPERIMENTS. 

The  fact  established  by  the  foregoing  experiments, 
that  one  colloid  (the  solid  jellies  being  nothing  else  but 
gelatinized  emulsoids)  is  practically  impermeable  by  an- 
other, leads  to  their  recognition  as  dialyzing  membranes. 
Dialysis  therefore,  is  intimately  connected  with  diffusion. 
Every  substance,  which  does  not  diffuse  appreciably,  but 
allows  itself  the  passage  of  crystalloids  may  be  used  as  a 
dialyzer,  and  inversely  we  might  say  "that  any  mem- 

1  Zeitschr.  f.  phys.  Chem.,  3,  p.  316,  (1889). 


124         DEMONSTRATIONS   IN    PHYSICAL   CHEMISTRY 

brane,  which  permits  the  passage  of  a  crystalloid  and  hin- 
ders the  passage  of  a  colloid,  is  itself  a  colloid."1 

Such  membranes  are  parchment,  (first  used  by  Graham 
in  1861)  fish-bladder,  urinary  bladder,  egg-membrane, 
parchment  paper  and  collodion  film  (in  sheet  or  tube 
form)  as  introduced  by  Malfitano  (1904). 

The  process  of  dialysis  requires  too  much  time  to  show 
marked  results  in  the  course  of  one  lecture  hour.  Be- 
sides the  classical  Graham  dialyzer  some  modern  types 
of  dialyzer  in  tube  form  (made  of  parchment  paper  or 
collodion)  and  the  new  "star-dialyzer,"  devised  by 
Zsigmondy  and  Heyer8  may  be  demonstrated. 

150.  To  show  the  extreme  facility,  with  which  crystal- 
loids pass  through  a  parchment  membrane,  a  solution  of 
potassium  thiocyanate  (KCNS)  is  put  inside  a  flat  Gra- 
ham   dialyzer,    which   is   then   left   floating   in    distilled 
water  for  I  or  2  minutes.     On  adding  ferric  chloride  to 
the  outside  water  a   red  color   appears,   demonstrating 
that   the   inside   crystalloid   passes   readily   through   the 
membrane.3     Placing  a  colloid  solution  of  prussian  blue 
into  a  second  parchment  dialyzer,  no  perceptible  trace  of 
a  blue  coloration  is  seen  in  the  outside  water,  not  even 
after  several  hours. 

151.  The   difference   in    efficiency    in   using   different 
membranes  may  be  conveniently  illustrated4  by  pouring 
a  moderately  concentrated  solution  of  fluorescem  into  a 
parchment  paper  tube  and  into  a  similar  collodion  tube. 

1  Bigelow,  1.  c.  p.  246. 

2  Zeitschr.  f.  anorg.  Chem.,  68,    p.  169,  (1910). 

3  Bigelow,  1.  c.  p.  244. 

4  Zsigmondy,  Kolloidchemie,  Leipzig,  p.  33   (1912). 


COI^OIDS   AND   ADSORPTION 


125 


Both  tubes  are  inserted  in  large  lecture  jars,  filled  with 
tap  water.  The  dye-stuff  diffuses  after  a  short  time 
through  the  collodion  membrane,  as  is  shown  by  the  rapid 
appearance  of  green  fluorescent  bands  in  the  outside 
water,  while  it  takes  a  considerable 
time  to  pass  through  the  parchment 
membrane. 

152.  The  collodion  tubes,  as  used 
by  many  colloid-workers  (Malfitano, 
Henry,  Duclaux,  Biltz,  Bigelow  and 
others)  are  made — as  is  easily  demon- 
strated in  the  lecture — by  sticking 
large  well-cleaned  bulb  test-tubes 
(Fig.  54)  into  solutions  of  collodion 
in  ether,  ether  and  alcohol,  or  acetic 
acid  and  water,  allowing  the  layer  to 
harden  in  the  air  and  repeating  the 
process  two  or  three  times  if  neces- 
sary, finally  hardening  the  whole  by 
washing  in  water.  The  collodion 
coating  is  then  cut  off  in  the  middle  of 
the  bulb  and  carefully  stripped  off. 
Details  may  be  looked  up  in  Bigelow 
and  Gemberling's  article1  on  "collod- 
ion membranes." 


Fig.  54. 


III.    VISCOSITY  EXPERIMENTS. 

153.  The  viscosity  of  suspensoids  does  not  perceptibly 
differ  from  that  of  pure  water ;  the  viscosity  of  emulsoids 
on  the  other  hand,  even  at  small  concentration  is  much 


Trmrn      A  tn 


(TOO7V 


126         DEMONSTRATIONS   IN    PHYSICAL   CHEMISTRY 

greater  than  that  of  water  and  increases  rapidly  on  cool- 
ing, while  suspensoids  do  not  assume  an  oily  or  even  a 
gelatinous  appearance  on  lowering  the  temperature.  This 
is  most  readily  shown1  by  allowing  10  cc.  of  a  2  per  cent, 
gelatin  solution  (an  emulsoid)  and  10  cc.  of  colloid  ar- 
senious  sulphide  (a  suspensoid)  to  flow  simultaneously 
from  two  10  cc.  pipettes  with  capillary  tips  which  are  as 
nearly  alike  as  possible.  The  time  of  outflow,  which  is 
— roughly  speaking — directly  proportional  to  the  fluidity 
or  inversely  proportional  to  the  viscosity,  is  much  longer 
for  the  emulsoid  solution.  Test-tubes  with  both  solu- 
tions, cooled  in  ice  water,  show  a  marked  difference; 
the  gelatin  solution  changes  into  a  thick  jelly;  the  sus- 
pensoid does  not  gelatinize  at  all. 

IV.  SURFACE;  TENSION. 

154.  From  careful  measurements  it  has  been  deduced, 
that  coarse  suspensions  and  suspensoids  hardly  alter  the 
surface  tension  (against  air)  of  the  dispersion  medium 
(water)  ;  emulsions  and  emulsoids  on  the  other  hand  de- 
crease the  surface  tension  of  their  dispersion  medium. 
This  difference,  therefore,  can  be  used  for  discriminating 
between  both  classes  of  colloids.2  The  decrease  of  sur- 
face tension  is  manifested  by  the  more  or  less  easy  forma- 
tion of  foam.  Thus  by  shaking  two  glass-stoppered  bot- 
tles, containing  arsenious  sulphide  sol  and  a  dilute  solu- 
tion of  Venetian  soap  (or  egg  albumen)  respectively,  only 
in  the  latter  case  an  abundant  foam  formation  is  ob- 

1  Ostwald-Fischer,  1.  c.  p.  13. 
-  Ostwald-Fischer,  1.  c.  p.  183. 


COLLOIDS   AND   ADSORPTION 


127 


served.     Instead  of  shaking  the  liquids,  an  indifferent 
gas,  like  nitrogen  or  air  may  be  bubbled  through. 

155.  Though  little  is  known  about  the  conditions  of 
stability  for  emulsoids,  it  can  be  shown  that  by  lowering 
the  surface  tension  of  two  un-miscible  liquids  against 
each  other,  an  emulsion  is  readily  obtained.1    In  a  beaker 
with  water  a  thin  layer  of  olive  oil  is  poured.    By  stirring 
the     mixture,     an     emulsion 

forms,  which  disappears  rap- 
idly. If,  however,  a  few 
drops  of  potassium  or  sodium 
hydroxide  are  added,  a  milky 
emulsion  is  formed  on  stir- 
ring, which  does  not  notice- 
ably change,  even  after  sev- 
eral hours. 

156.  This  decrease  in  sur- 
face,   (or  better,  boundary-) 
tension  can  be  followed  on  a 
measuring  scale  with  the  aid 
of  Donnan's    pipette    and  is 
most  suitably  made  visible  to 
a  large  audience    by    projec- 
tion on  the  screen.2 

The  pipette,  used  for  this 
purpose  (Fig.  55),  is  pro- 
vided with  a  stopcock  and  a 
capillary  outflow,  bent  upward.  It  is  filled  with  olive 

1  Donnan,  Zeitschr.  f.  phys.  Chem.,  31,  p.  42,  (1899). 

2  Donnan,  1.  c.  p.  42,  cf.  Hatschek,  1.  c.  p.  38  ;  and  Kruyt,  Chern.  Weekbl., 
10,  P-53°,  (1913)- 


Fig.  55. 


128         DEMONSTRATIONS   IN    PHYSICAL   CHEMISTRY 

oil  (containing  free  fatty  acid)  or  paraffin  oil,  to 
which  a  small  amount  of  fatty  acid  (palmitic  or  stearic 
acid)  has  been  added.  By  carefully  opening  the  stop- 
cock, the  oil  is  allowed  to  slowly  escape  in  water  writh 
the  formation  of  well-shaped  spherical  drops  which  rise 
to  the  surface  (10-20  drops  in  a  minute).  If  now  the 
water  is  replaced  by  a  solution  containing  a  few  drops 
of  a  base  (sodium  or  potassium  hydroxide),  the  number 
of  oil  drops,  formed  in  one  minute  is  more  than  doubled 
or  even  changes  into  a  continuous  stream  of  small  drop- 
lets, the  rapid  succession  not  allowing  the  number  of 
drops  to  be  counted. 

Both  phenomena  may  be  explained  on  the  basis  of 
Willard  Gibbs'  theorem,  stating  that  substances,  which 
lower  the  surface  tension  of  the  dispersion  medium,  tend 
to  collect  in  its  surface.  We  must,  therefore,  assume, 
that  the  emulsification  is  caused  by  the  strong  superficial 
adhesion  of  the  soap  formed  by  the  oil. 

D.  Optical  Properties  of  Dispersoids. 

157.  Differences  between  true  solutions  and  dispersoids 
become  visible  by  exposing  the  liquids  to  be  examined  to 
the  light  of  a  powerful  incandescent  lamp  (arc-light),  or 
still  better,  to  a  beam  of  sunlight  entering  the  darkened 
lecture  room  through  a  hole  in  the  window  shutter 
Usually  a  condenser  with  diaphragm  is  used  to  concen- 
trate the  light  on  the  liquid,  contained  in  a  beaker  or  a 
Dewar  tube  (in  case  hot  or  very  cold  liquids  are  tested). 
The  heterogeneity  of  colloid  solutions  is  then  easily  rec- 
ognized by  a  more  or  less  opaque  cone  of  light,  caused 


COLLOIDS   AND   ADSORPTION  129 

by  the  diffuse  reflection  of  light  from  the  discrete  par- 
ticles present  in  the  liquid.  This  is  the  so-called  "Tyndall 
phenomenon."  It  can  be  differentiated  from  fluor- 
escence by  its  property  of  being  polarized.  Looking  at 
the  cone  through  a  Nicol  prism  the  cone  disappears, 
when  the  prism  is  turned  around  its  axis  over  a  certain 
angle.  Suitable  demonstration  liquids  are:  a  ferric 
chloride  solution,  a  dilute  ferric  hydroxide  sol  (prepara- 
tion, see  p.  120),  having  the  same  color,  a  gold  sol  (pre- 
paration, see  p.  118)  and  an  arsenious  sulphide  sol.  In 
the  case  of  the  ferric  chloride  solution  the  cone  is  hardly 
visible,  in  the  other  cases  Tyndall  cones  of  slightly  differ- 
ent turbidity  are  observed.1 

It  may  be  remarked  here,  that  some  crystalloid  solu- 
tions, as  for  instance  sugar,  show  a  faint  turbidity,  on  ap- 
plying the  Tyndall  test.  This  test,  therefore,  is  not  con- 
clusive in  dubious  cases. 

E.  Electrical  Properties  of  Dispersoids. 

The  most  striking  reactions  shown  by  colloid  solutions 
are  those  connected  with  their  electric  behavior.  The 
fact  that  many  substances  in  colloid  solution  assume  an 
electric  charge  towards  the  dispersion  medium  may  be 
illustrated  by  migration  experiments.  Taking  as  ex- 
amples two  typical  suspensoids  of  opposite  character  such 
as  silver  sol  and  ferric  hydroxide  sol.  With  these  two 
colloids  (preparation  see  pp.  117  and  120),  suitably 
dialyzed  before  use,  the  following  migration  experiment 
may  be  performed.2 

1  Noyes,  1.  c.  p.  96. 

*  Forster,  die  chemische  Industrie,  28,  p.  733,  (1905). 


I3O         DEMONSTRATIONS    IN    PHYSICAL   CHEMISTRY 


158.  A  U-shaped  tube  (length  of  limbs  about  10  centi- 
meters; diameter  1-1.5  centimeters)  is  half  filled  with 
silver  sol  and  covered  in  both  limbs  by  a  o.oi  per  cent, 
sodium  hydroxide  solution,  in  order  to  increase  the  stabil- 
ity of  the  sol.  A  ( second  tube  is  filled  with  ferric  hy- 
droxide sol  covered  by  a  o.oi  per  cent,  sodium  acetate  so- 
lution. For  these  and  similar  experiments  U-tubes  with 
two  stopcocks,  having  the  same  bore  as  the  inner  cross- 
section  of  the  tube,  as  devised  by  Coehn1  (Fig.  56),  are 
very  useful.  These  tubes  are 
half  filled  with  the  solutions  to 
be  used,  the  stopcocks  are  then 
closed,  the  excess  of  the  liquid 
poured  out,  the  upper  parts  of 
both  limbs  rinsed  with  distilled 
water  and  filled  to  the  same 
height  with  the  required  solution. 
Platinum  electrodes  are  then  in- 
serted, the  stopcocks  opened  and 
the  current  passed  through.  In 
the  above  mentioned  experiment 
both  U-tubes,  are  connected  in 
series  with  the  terminals  of  the 
22O-volt  direct  current  lighting  curcuit.  After  20-30 
minutes  it  will  be  seen,  that  the  silver  sol  has  moved 
towards  the  anode,  the  iron  hydroxide  sol  towards  the 
cathode.  No  actual  separation  on  the  electrodes  occurs, 
the  visible  effect  being  limited  to  a  more  or  less  dense 
cloud,  collecting  in  the  neighborhood  of  the  electrodes. 


Fig.  56. 


1  Zeitschr.  f.  Electrochemie,  15,  p.  653  (1909). 


CONOIDS   AND   ADSORPTION  131 

Disturbing  effects  like  convection  currents  often  affect  the 
phenomenon. 

159.  A  more  elaborate  arrangement,  insuring  very  good 
results,  for  the  demonstration  of  colloid  migration  in  an 
electric  field — sometimes  called  "electrophoresis"  or 
"cataphoresis," — was  given  by  Noyes.1 

The  suspensoids  used  for  this  purpose  were  arsenious 
sulphide  sol  and  ferric  hydroxide  sol.  The  latter  had 
been  prepared  by  adding  to  a  molal  ferric  chloride  solu- 
tion a  molal  ammonium  carbonate  solution  until  the  pre- 
cipitate on  each  addition  would  barely  dissolve.  Both 
sols  had  been  dialyzed  for  a  week,  first  against  distilled 
water  and  finally  against  the  purest  water  obtainable 
(conductivity  water).  This  was  done  to  remove  electro- 
lytes as  completely  as  possible,  in  order  to  avoid  convec- 
tion by  the  heat,  produced  by  the  current,  causing  disturb- 
ance of  the  moving  surface.  Two  U-tubes,  15  centimeters 
in  total  height  with  a  3-centimeter  bore  covered  at  both 
ends  with  goldbeaters'  skin,  are  completely  filled  by  pour- 
ing the  sols  through  a  hole  at  the  bottom  of  the  bend,  i 
centimeter  in  diameter,  closed  by  slipping  a  rubber  band 
over  it.  The  limbs  of  each  tube  are  surrounded  by  glass 
tubes  of  a  slightly  greater  diameter  and  fitted  tightly  by 
means  of  rubber  bands,  connecting  both  glass  walls.  These 
tubes  extend  about  5  centimeters  above  the  goldbeaters' 
skin  and  are  filled  with  conductivity  water  (Fig.  57). 
After  inserting  platinum  wire  electrodes,  the  tubes  are 
connected  in  parallel  with  the  terminals  of  a  no-volt  (if 
available  22O-volt)  direct  current  lighting  circuit  with  a 

1  Noyes,  1.  c.  p.  97. 


132         DEMONSTRATIONS    IN    PHYSICAL   CHEMISTRY 

copper  coulomb-meter  in  series  to  indicate  the  direction 
of  the  current.  After  5-10  minutes  the  ferric  hydroxide 
sol  is  seen  moving  downward  with  a  sharp  boundary  sur- 
face, leaving  clear  water  above,  towards  the  cathode, 


Fig.  57. 

while  the  arsenious  sulphide  moves  downward  towards 
the  anode,  thus  proving  that  the  former  possesses  a  posi- 
tive charge,  while  the  latter  is  negatively  charged. 

It  should  be  remembered,  that  this  phenomenon  is  not 
limited  to  suspensoids  (and  some  emulsoids  like  egg-al- 
bumen) but  is  also  observed  in  the  case  of  suspensions 


COLLOIDS   AND   ADSORPTION 


133 


like  kaolin,  quartz  and  lamp  black.  The  reverse  of  this 
motion  is  called  electrical  "endosmosis,"  and  was  dis- 
covered by  Reusz  (1807).  For  tne  sake  of  completeness 
two  experiments  demonstrating  this  electro-osmosis  may 
be  cited. 

160.  A  plug  of  cotton  is  tightly  pressed  into  the  bend 
of  a  U-tube,  and  the  tube  half  rilled  with  distilled  water.1 
Platinum  electrodes  are  inserted  and  connected  with  the 
terminals  of  the  lighting  circuit  (direct  current).       The 
water  is  seen  moving  towards  the  cathode,  as  shown  by 
the  rise  in  level  on  the  side  of 

the  negative  electrode.  Freund- 
lich2  replaces  the  cotton  by  a 
bundle  of  short  capillary  tubes, 
and  obtains  a  like  result. 

161.  An  interesting  modifica- 
tion of  the  foregoing  experiment 
is  the  following:  An    unglazed 
porcelain  plate,  covered  with  dis- 
tilled water  is  supported  on  an 
iron    ring,    connected    with    the 
negative  pole  of  the  lighting  cir- 
cuit  (220- volt,  direct    current), 
while  the  positive  pole  is  formed 
by  a  lead  disk,   dipping  in  the 
water.     (Fig.  58.)     As  soon  as 
the    current    is    turned    on,    the 

water  is  seen  dropping  from  the  plate,  a  flow,  which 
comes  to  a  standstill  on  disrupting  the  connection. 

1  Coehn  in  Muller-Pouillet's  Handbook,  p.  615. 

2  Kapillarchemie,  p.  224,  (1909). 


Fig.  58. 


134         DEMONSTRATIONS   IN    PHYSICAL   CHEMISTRY 

On  account  of  the  different  behavior  in  an  electric 
field  we  distinguish  between  negative  and  positive  col- 
loids. 

Examples  of  the  first  are,  besides  arsenious  sulphide: 
antimony  sulphide,  gold,  silver,  sulphur,  selenium,  prus- 
sian  blue,  etc.  Positive  colloids  are  metallic  oxides 
(iron,  aluminium,  etc.).  * 

162.  A  simple  method  to  discriminate  between  these 
two  types  of  colloids  without  electrophoresis  is  based  on 
the  capillary  analysis,  the  use  of  which  was  introduced 
about  40  years  ago  by  a  Swiss  chemist  Goppelsroeder, 
and  recently  extensively  applied  in  colloid  chemistry  by 
Fichter  and  Sahlbohm. 

The  experiment  consists  in  dipping  the  strips  of 
filter  paper  into  colloid  solutions  of  (i)  ferric  hydrox- 
ide and  (2)  prussian  blue.  In  the  latter  case  the  colloid 
ascends  along  with  the  water  up  the  strip  of  paper  over 
some  10-20  centimeters,  depending  on  the  kind  of  blotting 
paper  used,  while  the  ferric  hydroxide  shows  a  marked 
lag  in  rising.  The  dispersion  medium  (water)  rises  as 
high  as  in  the  case  of  the  prussian  blue  sol,  but  the 
colloid  phase  (ferric  hydroxide)  rises  only  slightly  above 
the  level  of  the  liquid,  concentrates  then  and  finally  co- 
agulates.1 

A  third  method  to  determine  the  charge  of  an  un- 
known colloid  in  solution  is  to  test  it  with  two  solutions 
of  known  character,  such  as  ferric  hydroxide  sol  (+.) 
and  arsenious  sulphide  sol  ( — ). 

1  Ostwald-Fischer,  1.  c.  p.  15.  This  method  has  been  criticised  and  con- 
demned as  being  "  fallacious  "  by  Thomas  and  Garard,  (Journ.  Am.  Chem. 
Soc.,  40,  p.  joi,  1918).  It  is  therefore  advisable  to  check  the  results  of  the  "cap- 
illary analysis"  by  at  least  one  other  method. 


COI^OIDS   AND   ADSORPTION  135 

This  is  based  on  the  fact,  that  colloid  solutions  of  oppo- 
site charge  precipitate  each  other.  If  the  unknown  so- 
lution is  precipitated  by  arsenious  sulphide,  it  is  positive, 
and  in  case  ferric  hydroxide  sol  is  an  effective  precipitant, 
the  unknown  sol  is  negatively  charged. 

163.  In  the  lecture  the  formation  of  a  precipitate,  in 
bringing  together  these  two  typical  sols  (Fe(OH)3  and 
As^Sg)  may  be  carried  out. 

About  150  cc.  of  dialyzed  ferric  hydroxide  sol  and  200 
cc.  of  dialyzed  arsenious  sulphide-sol,  prepared  according 
to  the  foregoing  directions  (see  pp.  119  and  120)  are 
simultaneously  poured  into  a  lecture  jar;  a  flocculent 
precipitate  is  formed,  leaving  a  clear  solution  above.1 

164.  In  order  to  show  the  application  on  a  large  scale 
of  mutual  precipitation  of  colloid  solutions  of  opposite 
electric  charge,  it  is  interesting  to  test  for  this  purpose 
the  waste  water  of  some  industrial  plant,  which  usually 
forms  a  negative  colloid  solution.2     The  solution  is  fil- 
tered to  separate  particles  suspended  in  the  liquid.      On 
addition  of  the  required  quantity  of  a  dialyzed  colloid 
solution  of  ferric  hydroxide  of  known  strength,  a  precipi- 
tate is  formed,  which  settles  after  a  few  minutes,  leav- 
ing a  clear  solution  easily  separated  by  filtration  from 
the  precipitate.  The  quantity  of  ferric  hydroxide  sol  must 
correspond  with  the  so-called  optimum  of  precipitation,3 
and  has  to  be  found  out  by  trial-experiments  before  the 
lecture.     Taking  three  different  portions  of  the  test  so- 
lution, the  second  of  which  represents  about  the  required 

1  Noyes  1.  c.  p.  101. 

2  Baur,  1.  c.  p.  104. 

3  Biltz,  Ber.  d.  chem.  Ges.  37,  1095  (1904). 

10 


136         DEMONSTRATIONS    IN    PHYSICAL   CHEMISTRY 

amount,  the  effect  of  too  much  or  too  little  ferric  hydrox- 
ide sol  can  also  be  demonstrated. 

165.  Two   colloid   solutions    of    the    same    electrical 
charge  do  not  give  a  precipitate  on  mixing.       Thus  by 
bringing  together  200  cc.  of  an  arsenious  sulphide  sol 
and  200  cc.  of  a  gold-sol  no  precipitate  is  formed.    The 
gold-sol  is  previously  made  by  dialyzing  a  colloid  gold 
solution,  obtained  by  pouring  an  ethereal  solution  of  dry 
gold  chloride  into  an  aqueous  solution  of  acetylene.1 

166.  Precipitation  of  colloid  solutions  is  easily  brought 
about  by  the  addition  of  electrolytes.     Here  a  marked 
difference  between  emulsoids  and  suspensoids  must  be  em- 
phasized.    The  latter  are  most  readily   precipitated  by 
small  quantities  of  neutral  salts,  while  the  former  are 
not  precipitated  by  the  addition  of  salts,  unless  in  exces- 
sive amounts. 

This  is  shown  by  adding  10  cc.  of  a  normal 
solution  of  magnesium  chloride  to  each  of  two  test- 
tubes,  half  filled  with  a  i  per  cent,  gelatin  solution 
and  a  colloid  solution  of  arsenious  sulphide  respec- 
tively. The  first  solution  is  not  changed  apparently, 
while  the  second  shows  a  voluminous  precipitate.2 

167.  The  influence  of  the  valence  of  the  precipitating 
ion,  as  proved  by  experiments  of  Freundlicrr  and  others, 
is  very  pronounced;  and  was  demonstrated  by  Noyes4  in 
the  following  manner : 

1  Noyes,  1.  c.  p.  101. 

2  Noyes,  1.  c.  p.  101. 

3  Zeitschr.  f.  phys   Chem.,  44,  135,  151,  (1903). 
*  Noyes,  1.  c.  p.  102. 


CONOIDS   AND   ADSORPTION  137 

A  colloid  solution  of  (negative)  arsenious  sulphide 
is  made  by  mixing  equal  volumes  of  a  i  per  cent,  solution 
of  arsenious  oxide  and  a  saturated  hydrogen  sulphide 
solution  and  filtering  the  resulting  liquid. 

Fifty  cubic  centimeters  of  this  sol  are  poured  into  each 
of  seven  conical  lecture  jars,  containing  200  cc.  of  the 
following  solutions,  having  in  I  liter  dissolved : 

(1)  0.6  milli-equivalent     of  A1C13.* 

(2)  1.5  milli-equivalents  of  MgCl2. 

(3)  20.0  milli-equivalents  of  MgCl,.* 

(4)  60.0  milli-equivalents  of  NaCl. 

(5)  400.0  milli-equivalents  of  NaCl* 

(6)  60.0  milli-equivalents  of  Na2SO4. 

(7)  400.0  milli-equivalents  of  Na2SO4.* 

Coagulation  depends  here  on  the  cation,  the  Al*"-ion 
having  the  strongest  effect  and  the  monovalent  Na'-ion 
the  least.  The  Mg'Mon  occupies  an  intermediate  posi- 
tion. Precipitation  only  occurs  in  the  cases  marked  with 
an  asterisk. 

168.  Taking  a  positive  colloid  like  ferric  hydroxide 
sol,  it  is  the  anion,  that  causes  precipitation,  trivalent 
anions  having  the  greatest  effect,  monovalent  anions  the 
smallest,  insofar  that  then  the  greatest  quantity  of  salt  is 
required  to  bring  about  precipitation. 

To  eight  conical  lecture  jars,  containing  respectively 
200  cc.  of  solutions  having  in  I  liter : 

(1)  0.02  milli-equivalent     of  K3(FeCy6). 

(2)  o.io  milli-equivalent     of  K3(FeCy6).* 

(3)  o.io  milli-equivalent    of  Na2SO4. 


138         DEMONSTRATIONS   IN    PHYSICAL   CHEMISTRY 

(4)  i. 60  milli-equivalents  of  Na2SO4.* 

(5)  5.00  milli-equivalents  of  NaCl. 

(6)  50.00  milli-equivalents  of  NaCl.* 

(7)  5.00  milli-equivalents  of  MgCl2. 

(8)  50.00  milli-equivalents  of  MgCl2.* 

is  added  50  cc.  of  the  ferric  hydroxide  sol,  whereupon 
precipitation  is  observed  in  the  cases,  marked  by  an 
asterisk. 

169.  "Protective  Colloids."— The  use  of  emulsoids  in 
preventing  the   precipitation   of    dispersoids   is    demon- 
strated as   follows:1     Adding  first  200  cc.   of  a   N/5O 
sodium  chloride  solution  to  200  cc.  of  a  N/5O  silver  ni- 
trate  solution,    containing   5    cc.   of    strong   nitric   acid 
(specific  gravity  1.42),  a  white  flocculous  precipitate  im- 
mediately forms. 

The  experiment  is  then  repeated  with  equally  strong 
solutions  of  both  salts,  containing  I  per  cent,  of  gelatin 
dissolved.  The  mixture  becomes  opalescent,  and  the 
turbidity  increases  after  a  while,  without  forming  a  pre- 
cipitate. 

170.  The  deflocculation  of  suspensions  by  the  addition 
of  a  small  amount  of  acid  and  the  stabilizing  effect  of 
hydroxyl-ions  are  readily  demonstrated  as  follows: 

Ordinary  China  clay  is  stirred  up  in  water,  so  as 
to  form  a  suspension,  which  settles  out  rather  quickly, 
leaving  a  clear  liquid  above  and  a  sharply  defined  sedi- 
ment below.  If,  however,  a  little  alkali,  or  a  salt  with 
alkaline  reaction  is  added,  it  will  be  observed  that  the 

1  Noyes,  1.  c.  p.  91. 


CONOIDS   AND   ADSORPTION  139 

settling  takes  place  much  more  slowly,  the  smallest  par- 
ticles not  settling  out  at  all,  or  if  so  only  very  gradually. 

171.  The  mobility  of  a  clay  suspension  containing  a 
little  acid  is  very  much  less  than  that  of  the  same  sus- 
pension with  a  trace  of  alkali  as  may  be  shown  by  allow- 
ing the  suspensions  (which  must  be  rather  concentrated) 
to  flow  down  an  inclined  glass  plate. 

172.  With  a  suspension  of  colophony  (rosin)  the  de- 
flocculation   by   one    drop   of    acid   is    a   very    striking 
phenomenon.      An   opaque  suspension   of   a  milky  ap- 
pearance is  obtained  by  dissolving  0.5  gram  rosin  in  10 
cc.  of  alcohol  and  pouring  the  solution  in  90  cc.  water. 
On  adding  one  drop  of  5N  hydrochloric  acid  an  immedi- 
ate deflocculation  takes  place.    A  small  amount  of  alkali 
dissolves  the  flocks  with  the  formation  of  a  soap. 

F.  Adsorption. 

Adsorption  includes  a  number  of  closely  related 
phenomena,  sometimes  distinguished  as  (i)  adsorption, 
(2)  absorption,  occlusion  or  solution  and  (3)  formation 
of  absorption  compounds.  A  sharp  demarcation  between 
these  groups  is  impossible.  In  some  cases,  e.  g.,  that  of 
palladium,  taking  up  hydrogen,  it  is  likely  that  all  three 
phenomena  occur.  In  order  to  avoid  these  cumbrous 
distinctions  some  authors  speak  of  "sorption."  The  fol- 
lowing mostly  well-known  experiments  on  sorption  or, 
— using  the  more  familiar  term  adsorption  as  a  general 
designation — on  adsorption  refer  to  the  condensation  of 
(a)  gases,  (b)  liquids,  and  (c)  dissolved  substances  on 
different  solids. 


140         DEMONSTRATIONS    IN    PHYSICAL   CHEMISTRY 

173.  Coses. — Twenty   to   thirty    cubic   centimeters   of 
dry  ammonia  gas  are  collected  over  mercury  in  a  eudiom- 
eter tube.     A  small  piece  of  charcoal, — preferably  co- 
coanut  charcoal, — previously  heated  over  a  Bunsen  flame 
to  expel  adsorbed  gases,  on  coming  in  contact  with  the 
gas,    immediately    takes    up    several    cubic    centimeters, 
thereby    causing    a    considerable    rise    of    the    mercury 
column. 

174.  The   usefulness   of   charcoal   as   a   deodorant    is 
demonstrated  by  passing  a  slow  stream  of  hydrogen  sul- 
phide, washed  with  distilled  water  and  dried  over  granu- 
lated calcium  chloride,  through  a  tube    (length   50-100 
centimeters,  diameter  2  centimeters)  filled  with  previously 
ignited  wood  charcoal.     The  tube  is  connected  by  means 
of  an~|-shaped  delivery  tube  with  a  lecture  jar  containing 
a  lead  acetate  solution.    No  blackening  is  seen. 

175.  The  fact  that  adsorption  is  accompanied  by  heat 
evolution,  accounts   (partly)   for  the  following  phenom- 
enon.    A  piece  of  platinum  foil,  heated  over  a  Bunsen 
flame,  is  allowed  to  cool  by  turning  off.  the  gas,  until  the 
foil  is  no  longer  red  hot.    The  gas  is  then  turned  on  again, 
causing  the  platinum  to  glow  stronger  and  stronger,  until 
finally  the  gas  is  relighted.     Adsorption  and  combination 
heat  accumulate  here  in  raising  the  temperature  to  the 
ignition  point. 

If  adsorption  involves  heat  evolution,  lowering  of  the 
temperature  must  increase  the  quantity  of  absorbed  gas. 
Numerous  experiments  by  Dewar  and  others  have  cor- 
roborated this  conclusion  and  an  ingenious  method  of 
creating  a  high  vacuum  was  based  hereon.  (See  Chap- 
ter XII.) 


COLLOIDS   AND   ADSORPTION  14! 

176.  Liquids. — The  adsorption  of  water  by  charcoal, 
powdered  clay,  kaolin,  silica  and  other  finely  divided  ma- 
terials is  illustrated  by  heating  5-10  grams  of  the  sub- 

nee  in  a  test-tube.     Water  will  be  seen  to  condense 
nst  the  upper  walls  of  the  tube. 

177.  Dissolved  Substances. — The  adsorptive  power  of 
amorphous  carbon  in  the   form  of  wood  charcoal,  bone 
black,  blood  charcoal,  etc.,  has  been  so  universally  rec- 
ognized, that  only  quite  recently  other  equally  effective  and 

a  per  substances  have  come  into  use.  In  the  sugar  in- 
dustry animal  charcoal  has  been  replaced  by  mixtures 
of  wood  meal  and  Fuller's  earth:  in  the  oil  industries  the 
last  named  substance  has  been  lately  introduced  for  de- 
colorizing oils  ;  iu  purifying  potable  waters  use  is  made  of 
the  flocculent  precipitates  formed  by  aluminum  and  iron 
salts,  etc.  The  difference  in  adsorptive  power  of  various 
adsorbentia  may  be  exemplified  by  adding  to  five  2OO  cc. 
Erlenmeyer  flasks,  each  containing  100  cc.  of  a  dilute  con- 
go-red  solution  (o.i  gram  in  i  liter  water) :  i  gram  of 
talcum  powder,  i  -ram  of  (English)  Fuller's  earth,  I 
gram  of  finely  divided  bone  black,  10  cc.  of  alumina  cream 
and  10  cc.  of  ferric  hydroxide  cream  respectively.  The 
aluminium  and  iron  hydroxide  paste  are  made  by  precipi- 

ing  dilute  solutions  of  aluminium  and  ferric  chloride 
with  an  CXCCSS  of  ammonia,  decanting  the  supernatant 
liquid  and  frequent  washing  of  the  flocculent  precipitates 
with  distilled  water.  The  pastes  should  contain  in  IO  cc. 
about  0.6 — O.8  grain  of  the  anhydrous  oxides.  On  heating 
the  flasks  over  a  Bunsen  flame  until  the  liquids  boil  and 
subsequent  filtering,  it  will  be  seen  that  the  live  filtered  so- 


142         DEMONSTRATIONS   IN    PHYSICAL   CHEMISTRY 

lutions  show  different  degrees  of  decoloration,  compared 
with  the  original  solution.  The  talcum  is  only  slightly 
colored,  and  the  shade  of  color  of  the  liquid  is  only  little 
lighter  than  that  of  the  original  solution;  the  Fuller's 
earth  shows  a  better  result,  while  the  solution,  treated 
with  bone  black  retains  a  faint  red  color.  The  fourth 
and  the  fifth  solution,  however,  are  completely  decolor- 
ized, and  the  alumina  cream  precipitate  on  the  filter  shows 
the  color  of  the  congo-red  very  distinctly.  Similar  re- 
sults may  be  obtained  by  using  other  organic  dye-stuff 
solutions  (e.g.  litmus,  indigo,  etc.). 

178.  With  filter  paper  the  two  following  interesting  ad- 
sorption experiments  can  be  performed.    A  few  drops  of 
a  barium  hydroxide  solution  are  allowed  to  fall  on  one 
spot  of  a  piece  of  filter  paper.    At  2-3  centimeters  dis- 
tance from  this  spot,  outside  the  wet  ring  is  put  0.2-0.5  cc- 
of  a  i  per  cent,  alcoholic  phenolphthalein  solution.    The 
red  color  does  not  appear  until  the  wet  rings  have  over- 
lapped each  other  over  some  distance,  thus  clearly  show- 
ing that  the  dissolved  substances  are  absorbed  by  the 
paper.1    Therefore,  the  first  5-10  cc.  of  a  filtrate  should 
be  rejected,  when  solutions  of  a  definite  strength  are  re- 
quired. 

179.  Differences  in  the  degree  of  adsorption  are  shown 
by  allowing  solutions  of  hydrochloric  acid  and  barium  hy- 
droxide of  the  same  strength  to  creep  along  strips  of  filter 
paper,  dipped  with  their  lower  end  into  the  solutions. 
After  the  liquids  have  been  sucked  up  as  high  as  5-10 
centimeters,  the  wet  portions  of  both  strips  are  tested  by 

1  Bigelow,  1.  c.  p.  241. 


AND   ADSORPTION  143 

touching  at  different  heights  with  glass  rods,  moistened 
with  methyl  orange  and  phenolphthalein  respectively.  It 
will  be  seen,  that  the  base  has  travelled  only  one-third  as 
far  into  the  paper  as  the  acid  which  has  gone  up  almost 
as  far  as  the  water  itself.1 

180.  The  process  of  dyeing  is  largely  one  of  adsorption 
by   the    animal    or    vegetable    fiber.      On   bringing    150 
grams  of  woolen  yarn  into  a  large  beaker,  containing  30 
milligrams   of   crystal   violet   in   two   liters   of   distilled 
water,  the  solution  is  practically  decolorized,  the  dye-stuff 
having  been  completely  adsorbed  by  the  wool. 

181.  A  piece  of  cotton  fabric,  dipped  in  a  dilute  solu- 
tion of  purified  congo-red,  which  is  a  direct  dyeing  cot- 
tion  dye-stuff,  no  mordant  being  required  for  "fixing" 
the  color,  takes  on  a  fairly  light  shade  of  red  color.    On 
adding  sodium  chloride,  or  still  better  Glauber's  salt  to 
the  solution,  and  dipping  another  piece  of  cotton  into  the 
liquid,  the  fabric  takes  on  a  much  deeper  tinge  of  red, 
thus  showing  the  marked  effect  of  salt  in  driving  the 
color,  uniformly  distributed  (German:  "egalisiert")  on  to 
the  fabric.    The  dye,  being  a  sodium  salt  of  a  complex 
organic  acid   (Na2C32H22N2S2O6),  is  of  colloid  nature, 
and  "salted  out"  within  the  fibers  of  the  fabric  by  the  inor- 
ganic salt,  thus  materially  assisting  in  the  process  of  ad- 
sorption by  the  cotton. 

On  bringing  the  fabric  in  a  beaker  with  hot  water,  part 
of  the  color  is  lost;  the  cotton  "bleeds." 

182.  An  interesting  phenomenon  is  the  "selective  ad- 
sorption" of  fine  powders,  suspensions  and  suspensoids. 

1  Ostwald-McGowan,  1.  c.  p.  229. 


144         DEMONSTRATIONS   IN    PHYSICAL   CHEMISTRY 

It  has  been  found  for  instance  that,  if  a  solution  of  po- 
tassium chloride  is  shaken  with  clay  and  poured  on  a 
filter,  part  of  the  potassium  is  missing  in  the  nitrate, 
while  all  the  chlorine  passes  through  the  filter.1 

Van  Bemmelen's  well-known  experiment,  showing  the 
strong  selective  absorption  power,  which  Fremy's  vol- 
uminous manganese  peroxide  exerts  on  potassium  sul- 
phate is  a  typical  instance.2 

The  manganese  peroxide  is  made,  according  to  Fre- 
my's directions3  by  adding  a  cold  mixture  of  150  grams 
of  water  and  500  grams  of  concentrated  sulphuric  acid 
to  100  grams  of  potassium  permanganate.  The  result- 
ing acid  is  slowly  decomposed,  in  the  course  of  2-3  days, 
with  evolution  of  oxygen.  After  frequent  shaking  with 
fresh  quantities  of  distilled  water,  a  powder  results, 
which  when  dried  in  air  has  the  average  composition  of 
MnO22H2O,  and  does  not  impart  an  acid  reaction  to 
water.  Twenty  grams  of  the  powder  are  suspended  in 
100  cc.  of  water  and  the  suspension,  mixed  with  100  cc. 
of  a  normal  solution  of  potassium  sulphate  (neutral 
towards  litmus),  shaken  for  some  time.  The  man- 
ganese peroxide  is  allowed  to  settle  and  the  supernatant 
liquid  filtered  and  tested  with  blue  litmus.  The  solution 
shows  a  distinct  acid  reaction. 

183.  An  analogous  result  is  obtained,  when  a  sus- 
pensoid  like  colloid  arsenious  sulphide  is  precipitated 
by  a  potassium  chloride  solution,  as  was  first  observed 
by  Whitney  and  Ober.4 

1  H.  W.  Wiley,  Agricultural  analysis,  Vol.  i,  p.  127,  (1906). 

2  Journ.  f.  prakt.  Chemie,  N.  F.  23,  p.  342,  (1881). 
*  Comptes  rendus  82,  p.  1232,  (1876). 

4  Journ.  Am.  Chem.  Soc.,  23,  p.  852,  (1901). 


CONOIDS   AND   ADSORPTION 


145 


TO 


On  adding  a  sufficient  amount  of  potassium  chloride 
(20  cc.  of  a  normal  solution)  to  100  cc.  of  dialyzed 
arsenious  sulphide  sol  (with  no  appreciable  acid  reac- 
tion, the  sol  is  precipitated,  absorbing  the  cation,  and 
the  supernatant  liquid  becomes  acid. 

184.  The  selective  absorption  power  of  soil  is  conven- 
iently demonstrated  with  the  aid 
of  an  apparatus,  devised  by 
Miiller.1  A  vertical  glass  cylin- 
der (Fig.  59),  75  centimeters 
long,  with  an  internal  diameter 
of  4.5  centimeters  is  closed  at 
each  end  by  a  perforated  rubber 
stopper,  provided  with  L,-shaped 
glass  trbes  for  the  passage  of  the 
solution  to  be  used.  The  lower 
part  of  the  cylinder  is  filled  with 
broken  glass  or  glass  beads,  cov- 
ered by  a  layer  of  glass  wool, 
about  i  centimeter  thick.  The 
cylinder  is  then  filled  up  with 
soil,  carefully  sampled,  air-dry 
and  previously  passed  through  a 
sieve.  The  soil  is  covered  with 
glass  wool.  The  standard  so- 
lution of  N/io  potassium  car-  Fig-  59. 
bonate,  contained  in  a  2  liter  bottle,  is  allowed  to  rise 
slowly  in  the  soil  and  is  gradually  deprived  from  its 
potassium,  the  latter  being  absorbed  by  the  soil.  The  so- 

1  Zeitschr.  f.  angew.  Chem.  13,  p.  501,  (1889). 


146         DEMONSTRATIONS    IN    PHYSICAL   CHEMISTRY 

lution,  finally  collected  in  the  lecture  jar  is  compared  with 
a  sample  of  the  original  solution,  collected  in  another  jar 
by  opening  a  pinchcock  (P^)  in  a  connecting  T-piece. 
The  flow  of  the  solution  through  the  soil  is  regulated  by 
a  screw  pinchcock  (P^.  If  the  experiment  is  properly 
carried  out,  nearly  all  the  potassium  is  adsorbed,  so  that 
the  final  solution  is  neutral  towards  red  litmus,  while  the 
original  solution  is  distinctly  alkaline. 

As  this  test  is  not  quite  satisfactory  for  showing  the  loss 
of  potassium,  a  solution  of  picric  acid,  saturated  at  room 
temperature  (not  exceeding  17°)  may  be  used  as  in- 
dicator. Taking  N/5,  N/io,  and  N/2O  solutions  of  po- 
tassium salts  (K^COg,  K2SO4,  KC1),  it  will  be  seen  that 
on  shaking  10  cc.  of  the  solution  with  10  cc.  of  the  picric 
acid  solution,  a  heavy  precipitate  is  formed  with  the  N/5 
solution,  a  slight  precipitate  with  the  N/io  solution,  and 
no  precipitate  at  all  with  the  N/2O  solution.  A  N/5  so- 
lution of  a  potassium  salt  may  be  suitably  used,  since 
more  than  half  of  the  potassium  will  be  adsorbed. 

The  following  data,  obtained  by  Huston,1  may  be  in- 
cluded, to  show  the  selective  action  of  the  soil :  Two 
hundred  and  fifty  cubic  centimeters  of  N/io  solutions  of 
sodium  phosphate,  potassium  chloride,  potassium  sulphate, 
ammonium  sulphate  and  sodium  nitrate,  when  treated  for 
48  hours  with  100  grams  of  soil  lost  by  adsorption  respec- 
tively: 0.259  gram  P2O5,  0.316  gram  K2O,  0.332  gram 
K2O,  0.096  gram  N  and  o.ooo  gram  N. 

185.  Finally  one  out  of  many  cases  of  reciprocal  ad- 
sorption of  colloids  may  be  mentioned.  Mutual  adsorp- 

1  Experim.  Station,  Purdue  Univ.,  Bull.  33,  p.  50. 


CONOIDS   AND   ADSORPTION  147 

tion  of  two  suspensoids  has  as  yet  not  been  observed ;  on 
the  other  hand  a  number  of  cases,  in  which  either  one  or 
both  colloids  are  emulsoids  have  been  studied.  The  ac- 
tion of  protective  colloids  is  probably  nothing  but  an  ad- 
sorption of  the  emulsoid  by  the  suspensoid.  As  an  ex- 
ample of  reciprocal  adsorption  compounds  Cassius'  gold 
purple,  may  be  prepared.  This  substance,  long  consid- 
ered as  a  chemical  compound  of  tin  oxide  and  aurous 
oxide  is  really  a  mixture  of  (suspensoid)  colloid  gold 
and  (emulsoid)  colloid  stannic  acid,  as  has  been  proved 
by  the  investigations  of  Zsigmondy1  and  his  pupils. 

It  is  usually  obtained,  as  is  easily  shown  in  a  lecture 
demonstration,  by  adding  a  solution  of  stannic  and  stan- 
nous  chloride  to  a  very  dilute  solution  of  gold  chloride. 
The  gold  purple  can,  however,  also  be  prepared  by  mixing 
colloid  solutions  of  gold  and  stannic  acid.  These  so- 
lutions have  to  be  made  up  beforehand  and  are  obtained 
as  follows :  The  gold  solution  is  made  according  to  Zsig- 
mondy's  directions,2  by  starting  with  100  cc.  of  pure 
water,  (redistilled  from  a  quartz  or  pyrex  flask,  using  a 
silver  or  tin  condenser)  to  which  are  added  25  cc.  of  a 
solution  containing  0.6  gram  auric  acid.  The  latter  is 
obtained  by  evaporating  a  solution  of  gold  in  aqua  regia. 
The  mixture  is  then  treated  with  3  cc.  of  a  N/5  solution 
of  potassium  carbonate  and  boiled.  Four  cubic  centimeters 
of  a  solution,  containing  one  part  of  freshly  distilled  for- 
maldehyde in  one  hundred  parts  of  water  is  poured  grad- 
ually and  with  frequent  stirring  in  the  boiling  liquid.  In 
this  manner  a  deep  red  or  purple  red  gold  solution  of 

1  lyieb.  Ann.  301,  p.  361,  (1898). 

2  Ibidem  301,  p.  30,  (1898). 


148         DEMONSTRATIONS   IN    PHYSICAL   CHEMISTRY 

great  stability  is  obtained.  The  colloid  stannic  acid  is 
easily  prepared  by  dissolving  2  grams  of  anhydrous  stan- 
nic chloride  in  3  liters  of  distilled  water.  On  mixing  both 
solutions,  no  change  in  color  is  observed,  not  even  after 
the  addition  of  a  few  drops  of  dilute  nitric  or  sulphuric 
acid,  but  on  boiling  the  same  gold  purple  is  obtained  as  In 
the  usual  procedure  of  reducing  gold  chloride  with  stan- 
nous  chloride. 


CHAPTER  X. 

ACTING-CHEMISTRY. 

Although  a  great  many  reactions  are  known,  which  are 
influenced  by  light,  our  knowledge  of  radiant  energy  as 
such  is  still  very  limited.  No  theory  connecting  a  multi- 
tude of  observations  and  forecasting  unknown  phenom- 
ena, thereby  stimulating  further  researches  in  this  im- 
portant branch  of  physical  chemistry  has  been  put  for- 
ward. In  spite  of  persistent  investigations,  especially  in 
organic  chemistry,  where  "light"  reactions  are  most  ob- 
vious, the  work  of  Ciamician  and  Silber,  Benrath, 
Plotnikov  and  others  has  not  led  to  any  far-reaching  gen- 
eralization. 

186  The  most  typical  case  of  photo-synthesis,  which 
has  formed  the  subject  of  exhaustive  researches  by  some 
of  the  most  famous  chemists  of  the  igth  century  (Ber- 
thollet,  Draper,  Bunsen  and  Roscoe),  is  the  combination 
of  hydrogen  and  chlorine.  The  reaction  takes  place  with 
explosive  rapidity  under  the  influence  of  bright  sunlight 
or  the  light  of  burning  magnesium  ribbon.  The  experi- 
ment, shown  in  first  courses  in  chemistry,  may  be  safely 
carried  out  by  filling, — in  diffuse  light, — small  thick- 
walled  medicine  bottles  of  100  cc.  contents  over  brine 
with  the  mixture  of  the  gases  in  equal  volumes,  keeping 
the  bottles  corked  in  the  dark  until  needed.  Using  glass 
screens  of  various  color  (yellow  and  red),  the  absorption 
of  the  actinic  rays  may  be  shown  in  addition. 

187.  A  reversal  of  the  photochemical  union  of  chlorine 
and  hydrogen  is  the  decomposition  of  hydrochloric  acid 


150         DEMONSTRATIONS   IN    PHYSICAL   CHEMISTRY 

under  the  influence  of  light.  Coehn  and  Wassiljewa1  per- 
form this  experiment  by  passing  the  gas  (prepared  from 
fused  sodium  chloride  and  sulphuric  acid),  free  from 
air,  through  a  quartz  tube  20  centimeters  long  and  0.5 
centimeters  in  diameter,  illuminated  by  a  Heraeus'  mer- 
cury quartz  lamp  at  a  distance  of  about  2  centimeters, 
into  a  narrow  glass  tube,  blackened  on  the  outside,  which 
is  inserted  in  a  flask  with  potassium  iodide  solution.  The 
hydrogen  gas,  not  absorbed  in  the  solution,  is  collected 
in  an  explosion  eudiometer  and  exploded  with  oxygen  in 
the  usual  way.  The  operator  in  the  immediate  neigh- 
borhood of  the  quartz  lamp  should  not  forget  to  protect 
the  eyes  with  blue  glasses. 

188.  As  a  common  type  of  actinometer,  Eder's  mer- 
curic oxalate  actinometer  may  be  mentioned.    The  light 
activity  is  measured  here  by  the  chemical  transformation, 
which  mercuric  oxalate  undergoes  when  exposed  to  light. 

A  solution  of  4  grams  of  crystallized  ammonium  oxa- 
late in  loo  cc.  of  distilled  water  is  added  to  a  5  per  cent 
mercuric  chloride  solution  and  the  clear  liquid  is  then  ex- 
posed to  arc  light.2  The  separation  of  white  crystals 
of  calomel  soon  becomes  visible ;  at  the  same  time  carbon 
dioxide  is  liberated: 
2  HgCl2  +  (NH4)2C2O4  =  2HgCl  +  2NH4C1  +  2  CO2 

By  measuring  the  gas  volume  or  by  weighing  the  pre- 
cipitate the  light  intensity  may  be  quantitatively  deter- 
mined. 

189.  The  same  solution  may  be  used  for  illustrating 
photochemical     extinction.       This     phenomenon,     also 

1  Berichte  d.  chem.  Ges.  42,  p.  3183,  (1909). 

2  Meldola,  the  Chemistry  of  Photography,  Condon,  p.  32,  (189:). 


ACTING-CHEMISTRY  !$! 

called, — after  its  discoverer, — the  law  of  Draper  (1841), 
serves  to  demonstrate,  that  photochemical  decomposition 
implies  absorption  of  the  chemically  active  rays.  For 
demonstration  purposes,1  two  glass  troughs  are  taken, 
with  parallel  sides,  at  least  I  inch  apart,  each  divided  by 
a  vertical  septum,  and  strapped  together  by  means  of  rub- 
ber bands.  A  mixture  of  mercuric  chloride  and  ammo- 
nium oxalate  solutions,  made  up  as  above  mentioned  is 
poured  in  three  of  the  four  cells,  the  second:  B  (Fig.  60), 
being  filled  with  distilled  water.  The  whole  system  is 
then  exposed  to  the  arc  light,  A-B  being  nearest  to  the 
light.  As  soon  as  the  contents  of  A  becomes  opalescent, 


A 

a 

C 

D 

Fig.  60. 

the  cells  are  disconnected  and  on  exhibiting  the  results  it 
will  be  observed,  that  while  D  has  become  opalescent  C 
has  not  appreciably  been  affected,  no  opalescence  being 
visible. 

190.  An  electro-chemical  actinometer  is  described  by 
Coehn,2  adapted  for  a  demonstration  from  experiments 
by  Gouy  and  Rigollot.3  Into  a  U-tube,  filled  with  a  I 
per  cent,  sodium  chloride  solution,  two  strips  of  copper 
foil,  I  centimeter  wide,  previously  heated  over  a  Bunsen 
flame  until  the  clean  surface  has  taken  on  a  uniform 
brown  color,  are  inserted,  and  connected  by  means  of  a 

1  Meldola,  1.  c.  p.  327. 

2  Miiller-Pouillet's  Handbook  of  Physics,  p.  598. 

3  Journal  de  Physique  (3)  6,  p.  520,  (1897). 

II 


152         DEMONSTRATIONS    IN    PHYSICAL   CHEMISTRY 


copper  wire  with  a  lecture  galvanometer  and  a  contact 
key.  Both  limbs  are  covered  by  black  cardboard  caps. 
On  closing  the  circuit  no  deviation  of  the  pointer  is  vis- 
ible, but  on  removing  one  of  the  caps  a  deviation  is  ob- 
served. On  lowering  the  cap  the  pointer  moves  back 
again.  Raising  of  the  other  cap  reverts  the  current. 

191.  The  chemistry  of  photography  covers  a  large  field 
of  highly  interesting  phenomena,  offering  a  number  of 
unsolved  scientific  problems.  One  of  the  most  important 
reactions,  which  has  been  and  is,  up  to  the  present  time, 
a  matter  of  controversy  among  chemists,  is  the  well- 
known  photo-decomposition  of  silver  chlo- 
ride and  the  accompanying  change  in  color. 
This  may  be  illustrated  by  placing  some 
moist  silver  chloride,  freshly  prepared,  on 
the  bottom  of  a  cylindrical  vessel,  closed  by 
a  rubber  stopper,  through  which  passes  a 
glass  rod,  carrying  a  strip  of  starch  iodide 
paper.1  The  chloride  is  exposed  for  about 
10  minutes  to  the  electric  light.  (Fig.  61.) 
It  will  be  seen  that  the  chloride  rapidly 
darkens,  while  at  the  same  time  the  paper 
becomes  intensely  blue. 


Fig.  61. 


192,  The  retarding  effect  of  mercuric  chloride  may  be 
shown  at  the  same  time  by  exposing,  in  another  vessel, 
freshly  prepared  and  washed  silver  chloride,  to  which  a 
few  drops  of  mercuric  chloride  have  been  added.  Ex- 
posure to  the  light  produces  no  visible  change  in  the  salt.2 

1  Meldola,  1.  c.  p.  66. 

2  Meldola,  1.  c.  p.  67. 


ACTING-CHEMISTRY  153 

193.  Other  inorganic  salts,  which  are  readily  affected 
by  light,  are  cuprous  and  thallous  chloride.    The  follow- 
ing experiment  with  cuprous  chloride,  due  to  Priwoznick,1 
is  easily  performed.     A  sheet  of  polished  copper  with  a 
perfectly  clean  surface,  is  immersed  in  a  photographic 
disk,  filled  with  a  concentrated  copper  chloride  solution 
(made  by  boiling  hydrochloric  acid  with  an  excess  of 
cupric  oxide),  until  it  is  uniformly  covered  with  a  thin 
grey  film.    After  5  minutes  the  plate  is  removed,  washed, 
drained  on  blotting  paper,  and  when  still  moist,  exposed 
under  a  design,  cut  in  black  rjaper,  for  about  10  minutes 
or  longer,  to  the  electric  light.    The  design  appears  photo- 
graphed on  the  plate,  the  exposed  portions  being  much 
darker  than  the  protected  parts. 

194.  Apart  from  the  pre-eminent  value  of  the  silver 
haloids  for  reproduction  purposes,  two  other  processes 
deserve  to  be  mentioned :   the  "blue  print"  and  the  "pig- 
ment" process.     The  former  may  be  carried  out  in  the 
following  manner:     A  sheet  of  drawing  paper  is  coated 
with  a  10  per  cent,  ferric  ammonium  citrate  solution  by 
floating  on  the  liquid  for  a  few  minutes  and  dried  in  the 
dark.     It  is  then  covered  with  a  piece  of  black  paper  in 
which  a  design  has  been  cut  out  and  exposed  to  direct 
sunlight  or  to  arc  light,  concentrated  by  means  of  a  lens, 
for  5  or  10  minutes.    Under  the  influence  of  the  light  the 
ferric  salt  is  reduced  by  the  organic  material  of  the  paper 
and  a  faint  image  becomes  visible.     By  brushing  a  so- 
lution of  potassium  ferric  cyanide  over  the  exposed  sur- 

1  Dingler's  Pol.  Journ.  221,  p.  38,  (1877.) 


154         DEMONSTRATIONS   IN    PHYSICAL   CHEMISTRY 

face,  the  pattern  is  developed  in  Turnbull's  blue.    Finally 
the  non-exposed  ferric  salt  is  washed  out  with  tap  water. 

195.  The  pigment  process  depends  on  the  fact,  that 
gelatin,  containing  some  potassium  bichromate,  is  sensi- 
tive to  light,  when  dry,  but  hardly  sensitive  when  wet. 
The  process  may  be  illustrated  by  exposing  a  sheet  of 
drawing  paper,  previously  coated  with  a  mixture  of  gela- 
tin and  potassium  bichromate  together  with  finely  divided 
carbon  (or  any  other  pigment  used  in  oil  painting)  and 
dried  in  the  dark,  under  a  negative  to  direct  sunlight  or  to 
arc  light  for  several  minutes.  On  the  exposed  parts,  the 
gelatin  is  rendered  almost  insoluble.  Consequently,  on 
washing  the  paper  in  warm  water,  a  picture  appears  in 
the  pigment,  held  by  the  undissolved  gelatin.1 

Actino-chemistry  does  not  only  treat  of  reactions,  in 
which  light  causes  chemical  changes,  but  also  includes  the 
converse  processes  of  chemical  reactions,  producing  radi- 
ant energy.  Here  we  are  with  regard  to  a  deeper  under- 
standing of  these  transformations  almost  completely  ig- 
norant, since  apart  from  the  phenomena  and  the  names 
customarily  given  to  them,  little  or  nothing  is  known 
about  the  fundamental  principles  governing  these  differ- 
ent cases  of  so-called  luminescence.  Next  to  thermo- 
luminescence  and  electro-luminescence  (the  light  emitted 
by  rarified  gases  with  the  aid  of  the  alternating  current 
of  an  induction  coil),  of  which  no  instances  need  to  be 
mentioned,  we  distinguish :  tribo-luminescence,  crystal- 
lization luminescence,  fluorescence  and  phosphorescence. 

1  Bigelow,  1.  c.  p.  515. 


ACTING-CHEMISTRY 


155 


196.  Tribo-luminescence    may    be    observed,    when    a 
bottle,  containing  uranium  nitrate  crystals,  is  shaken  vig- 
orously in  the  dark. 

197.  Another  substance  showing  a  marked  tribo-lumi- 
nescence  is  salophen  (acetyl  para-amidophenyl  salicylate 
or  C6H4OHCOOC6H4NHCOCH3).  For  a  demonstration 
in  the  lecture  room  two  test-tubes  of  slightly 

different  diameter  are  used,  so  that  the 
one  with  smaller  bore  can  be  pushed  in  the 
larger  tube  (Fig.  62).  If  about  i  gram 
powdered  salophen  is  placed  in  the  annular 
space  between  the  tubes  and  crushed  by 
rotating  one  tube  within  the  other,  an  in- 
tense glow  is  observed  in  the  dark.1  For 
individual  observation  a  number  of  these 
tubes,  filled  with  salophen  are  circulated 
among  the  audience. 

198.  Crystallization  luminescence  is  more 
difficult  to  observe.    It  is  usually  shown,  by 
shaking   in   the   darkened   lecture   room   a 
supersaturated  solution  of    arsenious    acid 
or  sodium  fluoride.    As  soon  as  crystalliza- 
tion sets  in  flashes  are  seen,  but  the  light  being  very  faint, 
the  phenomenon  is  difficult  to  observe  from  a  distance. 

199.  Fluorescence,  first  discovered  with  fluorspar,  from 
which    mineral    the    phenomenon    derives    its    name,    is 
characteristic  of  several  mineral  oils  and  is  exceedingly 
marked  with  dilute  solutions  of  fluorescein  or  cosin. 

1  Plotnikov,  Photochemische  Versuchstechnik,  Leipzig,  p.  235,  (1912), 
which  contains  a  large  number  of  lecture  experiments  on  the  subject  of 
actino-chemistry  (p.  190—279). 


Fig.  62. 


156         DEMONSTRATIONS   IN    PHYSICAL,   CHEMISTRY 

200.  It  may  also  be  seen,  by  exposing  a  card,  moistened 
with  a  quinine  sulphate  solution  in  the  violet  and  ultra- 
violet region  of  the  arc-light  spectrum  obtained  with  a 
quartz  or  flint-glass  prism. 

201.  Phosphorescence  derives  its  name  from  the  glow 
which  phosphorus  emits  in  contact  with  oxygen.     As  is 
well  known,  no  glowing  is  seen,  when  the  element  is  ex- 
posed to  pure  oxygen,  under  atmospheric  or  higher  press- 
ure.   On  reducing  the  pressure  below  a  certain  limit,  the 
glow  becomes  visible.    This  is  particularly  well  shown  by 
using  the  arrangement,  given  by  Newth,1  consisting  of  a 
glass  tube  (length  30-50  centimeters;  diameter  2.5  centi- 
meters), bent  upward  at  both  ends  and  provided  with  two 
stopcocks  (Fig.  63).    A  solution  of  yellow  phosphorus  is 


Fig.  63. 

made  by  gently  warming  a  few  pieces,  the  size  of  a  pea, 
in  a  conical  flask  with  olive  oil.  The  bottom  of  the  tube 
is  then  covered  with  a  layer  of  this  solution,  and  after 
expelling  the  air,  filled  with  oxygen.  No  glowing  is  seen, 
but  on  reducing  the  pressure  with  the  water-suction 
pump,  the  tube  becomes  luminous  over  its  entire  length. 
The  experiment  requires  darkening  of  the  lecture  room. 
202.  The  glowing  of  phosphorus  can  be  observed  even 
in  diffuse  daylight,  by  following  the  directions,  given  by 
Marino  and  Porlezza.2  The  authors  pass  carbon  dioxide 
through  a  saturated  sodium  bicarbonate  solution,  dry  it 

1  Newth,  1.  c.  p.  243. 

2  Gazz.  chim.  ital.  41,  (II),  p.  420,  (1911). 


ACTING-CHEMISTRY 


157 


in  a  calcium  chloride  tower  and  then  introduce  it  in  a 
hard  glass  tube  (2  centimeters  diameter),  in  which  red 
phosphorus  is  heated  over  three  wing-top  Bunsen  burn- 
ers (Fig.  64).  When  starting  the  experiment  the  phos- 
phorus is  heated  very  gradually,  while  at  the  same  time 
the  gas  current  passes  over  the  phosphorus  very  slowly, 


Fig.  64. 

until  all  traces  of  moisture  have  been  expelled.  An  L- 
shaped  delivery  tube  is  then  connected  with  the  open  end 
of  the  tube,  leading  down  to  the  bottom  of  a  large  2- 
liter  Florence  flask.  The  combustion  tube  is  then  heated 
up,  until  the  phosphorus  condenses  in  yellow  droplets  in 
the  delivery  tube.  The  gas  stream,  which  has  been  kept 
slow  for  a  while,  is  then  suddenly  increased.  Imme- 
diately a  beautiful  greenish  flame  appears,  while  the  flask 


158         DEMONSTRATIONS   IN    PHYSICAL   CHEMISTRY 

itself  shows,  in  its  lower  part  a  splendid  phosphores- 
cence. 

203.  What  is  sometimes  called  "chemiluminescence" 
may  be  seen  in  certain  chemical  reactions,  where  lumines- 
cence accompanies  the  reaction.  Thus,  on  adding  rapidly 
50  cc.  of  a  30  per  cent,  hydrogen  peroxide  solution  to  a 
mixture  of  35  cc.  of  a  50  per  cent,  potassium  carbonate 
solution,  35  cc.  of  a  10  per  cent,  pyrogallol  solution,  and 
35  cc.  of  a  35  per  cent,  formaldehyde  solution,1  vigorous 
foaming  accompanied  by  a  reddish  glow,  results. 

1  Trautz,  Zeitschr.  f.  Electrochemie  10,  p.  593,  (1904);  Zeitschr.  f.  phys. 
Chem.,  53,  p.  i,  (1905). 


CHAPTER  XL 


FLAME,  COMBUSTION  AND  EXPLOSION. 

From  the  time,  when  the  phlogiston  hypothesis  was 
universally  accepted  by  Priestley,  Scheele,  Bergmann  and 
other  prominent  chemists  of  the  eighteenth  century,  up 
to  the  recent  flame  gas  investigations  by  Haber,  Bone  and 
their  co-workers,  many  attempts  have  been  made  to  ar- 
rive at  a  clear  insight  into  the  nature  of  flames  and  the 
causes  of  their  luminosity.  Numerous  experiments  are 
known, — the  more  important  are  given  in  almost  any  text- 
book of  elementary  inorganic  chemistry, — but  up  to  the 
present  time  a  general  explanation,  covering  all  the  re- 
search work,  that  has  been  done  by  Davy,  Frankland, 
Heumann,  Smithells,  Lewes,  Bone  and  others  cannot  be 
given. 

In  the  following  a  number  of  experiments  will  be  men- 
tioned and  briefly  described,  illustrating: 

I.  Combustion  of  gases  in  general. 
II.  The    structure   and    chemical    reactions    of 
flames. 

III.  Luminosity  in  the  presence  of  solid  particles. 

IV.  The  separation  of  solids  from  flames. 
V.  Luminosity  without  solid  particles. 

VI.  Changes  in  luminosity. 
VII.  Explosion  and  its  prevention. 


l6o         DEMONSTRATIONS   IN    PHYSICAL   CHEMISTRY 


I.   Combustion  of  Gases  in  General. 

204.  A  flame,  defined  as  a  mass  of  glowing  gas,  re- 
quires a  medium  in  which  it  can  "burn,"  that  is  the  com- 
bustion must  be  supported  by  another  gas,  in  order  to  pro- 
duce a  combination  of  the  two  gases  with  evolution  of 
heat  and  light.  The  term  "combustible"  and  "supporter 

of  combustion"  are  interchange- 
able, however,  as  shown  by  the 
experiment  of  the  "reversed 
flame."1 

A  lamp-glass  (Fig.  65)  is 
closed  at  its  lower  end  by  a 
cork  stopper,  carrying  a  central 
tube  of  metal  or  glass,  I  centi- 
meter in  diameter  and  a  smaller 
side  tube  (inner  bore  2-3  milli- 
meters), through  which  coal  gas 
is  admitted.  The  lamp  chimney 
is  closed  at  the  top  by  a  per- 
forated asbestos  disk,  and  the 
hole  in  this  cover  closed  until 
the  lamp  is  filled  with  gas. 
After  2-3  minutes  the  gas  is  ig- 
nited at  the  bottom  of  the  central 
Fig>  65-  tube  and  the  hole  in  the  asbestos 

disk  slowly  opened.  The  flame  is  drawn  up  into  the  tube 
and  the  reversed  flame  appears  in  an  atmosphere  of  coal 
gas.  The  gas,  issuing  from  the  hole  at  the  top  is  ignited 
and  represents  the  ordinary  coal  gas  flame.  By  introduc- 

1  Waitha,  Ber.  d.  chem.  Ges.  4,  p.  91,  (1871). 


FI<AMF,,    COMBUSTION    AND   EXPLOSION  l6l 

ing  a  small  tube,  from  which  coal  gas  is  burned,  into  the 
central  tube,  both  flames  are  close  together,  the  one  en- 
veloping the  other. 

205.  The  same  inference  is  reached  by  passing  a  jet  of 
oxygen  into  an  inverted  glass  cylinder,  filled  with  hydro- 
gen and  ignited  at  the  lower  end.     The  ordinary  oxy- 
hydrogen  flame  is  obtained  by  lowering  a  jet  of  burning 
hydrogen  into  a  glass  cylinder  filled  with  oxygen. 

206.  Substances,  that  give  off  oxygen  readily,  may  be 
used  to  burn  oxygen  in  a  coal  gas  atmosphere.  In  the  flame 
of  coal  gas,  burning  from  a  lamp-glass,  covered  by  an 
asbestos  disk  with  a  hole  in  the  middle,  dry  chlorates 
of  potassium,  barium  and  strontium  are  fused  successive- 
ly on  a  deflagrating  spoon   until   oxygen   is   given   off. 
The  spoon  is  then  lowered  and  the  liberated  oxygen  burns 
with  a  brilliantly  colored  light. 

207.  That  oxygen  may  be  replaced,  either  as  a  "sup- 
porter of  combustion"  or  as  a  "combustible"  by  another 
gas,   chlorine    for    instance   is   illustrated   by   introduc- 
ing a  jet  of  burning  hydrogen  into  a  glass  jar,  filled  with 
chlorine.     In  a  similar    way    chlorine  can  be  made  to 
burn  in  an  atmosphere  of  hydrogen  by  the  use  of  the  ap- 
paratus for  the  "reversed"  flame.     Replacing  the  short 
central  tube  by  a  long  tube,  which  reaches  the  hole  in 
the  asbestos  disk,  from  which  hydrogen  burns,  the  chlor- 
ine is  introduced  through  this   central  tube  in  a   slow 
stream  and  ignites  on  issuing  from  the  opening.     The 
tube  is  then  drawn  down  carefully.     It  will  be  seen  that 
the  chlorine  continues  to  burn  with  a  copious  evolution 
of  hydrogen  chloride  vapor. 


l62         DEMONSTRATIONS   IN    PHYSICAL   CHEMISTRY 

II.  The  Structure  and  Chemical  Reactions  of  Flames. 

Taking  the  flame  of  illuminating  gas,  obtained  with  a 
Bunsen  or  Teclu  burner  as  an  instance,  it  will  be  noticed, 
that  the  non-luminous  flame  represents  an  inner  and  an 
outer  cone.  The  inner  cone  burns  with  a  bright  green 
color,  which  must  be  ascribed  to  luminescence  since  the 
temperature  of  this  cone,  under  a  strong  draught,  accord- 
ing to  Haber  and  Richardt1  is  only  about  1550°.  The 
mixture  of  gas  and  air  burns  in  the  inner  cone  to  carbon 
monoxide,  carbon  dioxide,  hydrogen  and  water  vapor, 
mixed  with  uncombined  nitrogen  and  is,  therefore,  noth- 
ing but  "a  water  gas,  diluted  with  nitrogen"  (Haber). 

In  the  outer  cone  (temperature  ca  1800°)  the  carbon 
monoxide  and  hydrogen  form  carbon  dioxide  and  water 
vapor.  The  gas  mixture  between  the  two  cones  does  not 
contain  any  oxygen,  consequently  it  is  inferred,  that 
"nothing  burns  in  the  flame."  Inside  the  inner  cone  we 
have  the  unburned  gas  mixture;  the  flame,  therefore,  is 
hollow.  From  the  experiments,  devised  to  prove  this, 
the  following  may  be  quoted : 

208.  Well  glazed  writing  paper,  dusted  on  the  upper 
side  with  mercuric  iodide,  asbestos  paper  or  thin  copper 
foil,  when  depressed  for  a  short  time  upon  the  flame,  at 
various  angles,  show  the  well-known  flame  figures.    (Fig. 
66.) 

209.  A  glass  tube,  about  10-15  centimeters  long,  with 
an  inner  bore  of  5  millimeters,  cut  off  at  an  angle  of  45° 
at  its  lower  end,  is  held  in  the  flame,  as  indicated  in 

1  Haber-I^amb,  Thermodynamics  of  Technical  Gas  Reactions  p.  301,  (1908). 


FLAME,    COMBUSTION    AND   EXPLOSION 


the  figure  (Fig.  67),  and  the  unburned  gas  lighted  at  the 
upper  end. 


/'  ,// 

5 


Fig.  66. 


Fig.  67. 


210.  A  pin  is  pushed  at  right  angles  through  a  match, 
about  i  or  2  centimeters  from  its  head  and  the  latter  thus 
supported  vertically  on  the  jet  of  the  burner.1    On  light- 
ing the  gas,  the  match-head  does  not  catch  fire.    The  ex- 
periment can  be  repeated  by  thrusting  a  match  quickly 
into  the  center  of  the  flame ;  only  the  middle  part  burns 
off  directly. 

211.  A    modification    of    the    foregoing    experiment 
is  obtained  by  connecting  the  gas  supply  with  the  stem 
of  a  small  funnel  (5-7  centimeters  in  diameter)  covered 
with  a  piece  of  fine  copper  wire  gauze.     Right  in  the 
center  of  the  gauze  a  number  of  match-heads  or  a  small 
heap  of  gunpowder  may  be  placed.     On  turning  on  the 
gas  and  igniting  the  latter  with  a  burning  taper,  held  over 

1  Hofmann,  Ber.  d.  chem.  Ges.  2,  p.  254,  (1869). 


164         DEMONSTRATIONS    IN    PHYSICAL   CHEMISTRY 


the  gauze,  the  inflammable  material  in  the  center  remains 
unconsumed. 

212.  The  device  which  made  it  possible  to  investigate 
the  chemical  composition  of  the  inter-conal  gas  in  the 
Bunsen  flame  is  the  flame-  or  cone-separator,  found  by 
Smithells  and  Ingle1  and  simultaneously  by  Teclu.2 
The   apparatus    (Fig.   68)    consists   of   a   glass   tube, 
40  centimeters  long  and  15  millimeters 
wide,  fitted  at  its  lower    end    either 
with  a  T-piece  for  the  inlet  of  gas  and 
air,  the  supply  of  each  being  regulated 
by  stopcocks  (or  screw  clips)  or  with 
a    one-hole    rubber    stopper,    through 
which  passes  the  mouth  of  a  Bunsen 
or  Teclu  burner.     This  glass  tube  is 
surrounded  by  a  second  glass  tube,  20 
centimeters    long    and    3    centimeters 
wide,  fitted  with  a  rubber  union  and 
one  (or  two)  asbestos  or  cork  rings, 
to  permit  the  outer  tube  to  be  easily 
slid  up  and  down.     The  inner  tube  is 
held  by  a  clamp  in  a  vertical  position. 
The  upper  ends  of  both  tubes  are  pro- 
vided   with    metal    ends    (preferably 
platinum,  but  aluminium  or  copper  foil  will  do  just  as 
well).     In  order  to  make  both  cones  visible  to  a  large 
audience,  the  metal  ends  are  moistened  with  a  sodium 
chloride     solution.       The     outer     tube     may     be     pro- 

1  Journ.  Chem.  Soc.,  61,  p.  204,  (1892);  Brit.  Ass.  Rep.,  lyeicester,  (1907)  p.  469. 

2  Journ.  f.  prakt.  Chemie  44,  p.  246,  (1891). 


Fig.  68. 


COMBUSTION    AND   EXPLOSION  165 

vided  with  a  small  side  tube  (closed  by  a  cork  stopper) 
to  show  how  in  practice  the  interconal  gas  can  be  drawn 
off  for  analysis  (Smithells,  Haber  and  others).  At  the 
beginning  of  the  experiment  both  tubes  are  adjusted  to 
the  same  level.  The  gas  is  turned  on,  the  air  being  com- 
pletely shut  off ;  the  air  is  then  gradually  admitted  and  the 
luminous  flame  changed  into  the  ordinary  non-luminous 
flame.  The  outer  tube  is  then  raised  and  the  flame  split 
in  two  cones,  the  outer  cone  ascending  with  the  wide 
tube,  the  inner  cone  continuing  to  burn  at  the  opening  of 
the  narrow  tube.  By  increasing  or  decreasing  the  sup- 
ply of  air  the  inner  cone  can  be  made  to  "strike  back" 
or  to  ascend  to  the  mouth  of  the  outer  tube,  in  which 
case  the  original  Bunsen  flame  is  restored. 

213.  The  presence  of  carbon  in  the  luminous  flame  of 
illuminating  gas  is  indicated  also  by  Soret's  optical  test. 
The  flame  placed  between  a  screen  and  a  strong  light 
causes  a  shadow  to  be  formed  on  the  screen.    No  shadow 
is  formed  by  the  flames  of  carbon  disulphide  and  burn- 
ing phosphorus. 

214.  The  view  of  a  preferential  combustion  of  hydro- 
gen, liberating  carbon,  which  should  cause  the  luminosity 
in  luminous  flames  of  hydrocarbons  has  been  given  up  in 
favor  of  the  conception  of  a  gradual  dissociation,  pre- 
ceding combustion.     In  fact,  intermediate  formation  of 
acetylene  can  be  shown  in  certain  cases,  for  instance  by 
drawing  off  the  gases  formed,  when  a  Bunsen  burner 
strikes  back  and  passing  the  gas  mixture  through  an  am- 
moniacal  solution  of  cuprous  chloride  (Fig.  69). 


l66         DEMONSTRATIONS   IN    PHYSICAL   CHEMISTRY 


Fig.  69. 

Another  case  is  that  of  air  burning  in  an  excess  of 
illuminating  gas ;  here  again  acetylene  is  formed  and  the 
formation  of  this  gas  shown  in  the  same  way  as  before. 

III.   Luminosity  in  the  Presence  of  Solid  Particles. 

215.  That  non-luminous  flames  become  luminous  in  the 
presence  of  solids  is  well-known.  The  easiest  illustration 
is  holding  a  thin  platinum  wire  in  the  colorless  flame  of 
hydrogen,  burning  from  a  platinum  jet.  Other  instances 
are  "Drummond's  lime-light"  being  an  oxy-hydrogen 
flame,  directed  on  a  piece  of  quicklime,  and  the  Welsbach 
lamp,  which  is  an  ordinary  Bunsen  burner,  giving  a  non- 
luminous  flame,  made  luminous  by  the  incandescent 
mantle  of  thorium  oxide  (ThO2)  mixed  with  about  I  per 
cent,  of  its  weight  cerium  oxide  (CeO2). 


FLAME,  COMBUSTION  AND  EXPLOSION 


i67 


216.  The  flame  of  burning  alcohol  which  is  almost  in- 
visible, can  be  made  luminous  by  passing  a  jet  of  chlorine 
into  an  Erlenmeyer  flask,  in  which  alcohol  is  boiled.    The 
flame,  burning  from  the  neck  becomes  luminous  by  the 
separation  of  carbon,  formed  by  the  decomposition  of  the 
alcohol  by  the  chlorine. 

217.  A  number  of  experiments  have  been  devised  show- 
ing the  "carburetion"    of    non-luminous 

flames  by  the  introduction  of  carbon  in 
the  flame.  This  is  usually  done  by  mix- 
ing the  gas  with  unsaturated  hydro- 
carbons, rich  in  carbon,  as  benzene  or 
acetylene.  The  hydrogen  from  a  Kipp 
generator  is  passed  through  a  calcium 
chloride  tower,  connected  with  a  U-tube, 
both  legs  of  which  are  provided  with  fish 
tail  tips  (Fig.  70).  A  plug  of  cotton 
soaked  with  benzene  is  inserted  in  the 
left  limb,  and  after  having  expelled  the 
air  completely,  both  tips  are  lighted.  The 
luminous  flame  appears  on  the  left  side, 
while  the  right  flame  is  almost  colorless. 

218.  The   enrichment   of   illuminating 
gas  with  acetylene  makes  the  colorless 
flame  of  the  Bunsen  burner  quite  lumi- 
nous and  is  conveniently  carried  out  by 

passing  the  gas  into  a  wide-mouthed  bottle,  introduced 
between  the  gas  outlet  and  the  Bunsen  burner,  and  half- 
way filled  with  water.    A  piece  of  calcium  carbide  is  held 
12 


70- 


1 68          DEMONSTRATIONS    IN    PHYSICAL   CHEMISTRY 


by  a  copper  wire,  as  shown  in  the  figure  (Fig.  71  ).1 
After  shaking  off  the  carbide  into 
the  water,  the  luminosity  of  the 
flame  will  be  considerably  increased 
for  several  minutes. 
IV.  The  Separation  of  Solids 

from  Flames. 

An  interesting  way  of  separating 
solid  particles  from  a  flame,  is  by 
the  formation  of  vortices,  of  which 
two  instances  are  given  by 
Newth.2 

219.  Coal  gas  is  passed  through  a 
T-shaped  tube,  connected  with  two 

glass  tubes  (6-8  millimeters  bore)  drawn  out  in  capillary 
jets,  held  by  clamps  and  inclined  at  such  an  angle,  that 
by  carefully  regulating  the  gas  stream  in  each  tube,  two 


Fig.  71. 


Fig.  72. 

horn-shaped  wings  of  carbon  vortices  are  formed  (Fig. 
72).  Draught  must  be  avoided,  as  the  phenomenon  is 
affected  by  the  slightest  air  currents. 

1  Baker,  Journ.  Am.  Chem.  Soc.,   39     p.  646,    (1917). 

2  Newth,  1.  c.  p.  216. 


FLAME,    COMBUSTION    AND   EXPLOSION 


169 


220.  Much  easier  to  set  up  is  another  arrangement,  in 
which  an  Argand  burner  and  a  blowpipe  are  used.  The 
chimney  of  the  burner  (about  4  inches  high)  is  covered 
with  a  piece  of  twenty-five-mesh  wire  gauze  and  the  jet 
of  a  blowpipe  pressed  against  the  gauze  (Fig.  73).  The 
burner  is  lighted  and  the  flame  turned  down  as  far  as 
possible.  Coal  gas  is  then  admitted  through  the  blow- 


gas 


Fig.  73. 

pipe,  and  a  beautiful  vortex  ring  of  sparkling  carbon  par- 
ticles will  be  formed. 

221.  In  many  cases  the  substance,  to  which  the  lumin- 
osity is  due  can  be  separated  by  cooling  the  flame  with  a 
cold  object.  Thus  by  holding  a  porcelain  dish  over  the 
flame  of  illuminating  gas,  arsine,  stibine  and  nickel  car- 
bonyl  respectively,  the  separation  of  carbon,  arsenic, 
antimony  and  nickel  will  be  observed. 


I/O         DEMONSTRATIONS    IN    PHYSICAL    CHEMISTRY 


222.  In   the   following   experiment   the   separation   of 
heavy   metals    from   the   flames   may   be   demonstrated, 
by  following  the  example  given  by  Ste.  Claire  Deville1  in 
his  cold-warm  tube.     The  arrangement,  as  proposed  by 
Bancroft  and  Weiser2  consists  of  a  Bunsen  burner,  pro- 
vided with  an  asbestos  chimney   (Fig.  74).     The  latter 

has  a  hole,  through  which 
passes  a  platinum  wire,  bent 
in  a  loop,  covered  with  a  piece 
D  of  asbestos  soaked  in  a  con- 
centrated solution  of  an  easily 
volatilized  metal  salt  (like 
cadmium  chloride,  bismuth 
nitrate,  lead  nitrate).  After 
introducing  the  salt  in  the 
Fig.  74.  hottest  part  of  the  flame,  a 

porcelain  tube  of  about  I  centimeter  outer  diameter, 
cooled  by  a  rapid  stream  of  cold  tap  water,  is  held  for  a 
few  minutes  in  the  upper  part  of  the  flame.  The  result 
is  a  separation  of  the  metal, — very  often  in  the  form  of 
an  extremely  lustrous  mirror. 

V.   Luminosity  without  Solid  Particles. 

223.  Vapor  of  carbon  bisulphide,  burning  in  nitric  ox- 
ide, produces  an  intense  blue  light,  although  no  solid  par- 
ticles are  present.    In  a  jar,  filled  with  nitric  oxide  (over 
water),  a  few  drops  of  carbon  disulphide  are  introduced 
from  a  dropping  funnel,  and  after  thorough  shaking,  the 
mixture  is  ignited. 

1  I«e£ons  sur  la  dissociation,  Paris,  pp.  45-63,  (1864). 

2  Jour n.  of  phys.  Chem.,  18,  p.  213,  (1914). 


COMBUSTION    AND   EXPLOSION 


171 


8 


224.  On  burning  phosphine  in  pure  oxygen  (Fig.  75), 
the  gas  burns  with  a  dazzl- 
ing white  flame. 

VI.   Changes  in  Luminosity, 

The  luminosity  of  flames 
can  be  changed  either  by  in- 
creasing or  decreasing  the 
pressure  or  by  raising  or 
lowering  the  temperature. 
The  following  set  of  ex- 
periments illustrates  this  "nnr""  Of 
point : 

225.  Cooling  of  the  flame 
diminishes  the   luminosity: 

the  luminous  flame  of  a  Bunsen  or  a  fish-tail  burner  be- 
comes practically  colorless  by  holding  a  platinum  disk  or 
a  sheet  of  nickel  or  iron  plate  against  the  flame.  The 
luminosity  can  be  restored  by  heating  the  disk  or  sheet 
by  means  of  a  blast  lamp  on  the  opposite  side. 

226.  Liipke1  illustrates  the  effect  of  a  cooling  in  the 
following  manner.     A  10  per  cent,  solution  of  ether  in 
water  is  poured  in  a  test-tube  and  the  tube  closed  with 
a  cork  stopper,  through  which  a  needle  is  run  down  to 
the  bottom.    The  solution  is  solidified  in  a  freezing  mix- 
ture, the  test-tube  stripped  off  and  the  frozen  cylinder 
fixed  upside  down  in  a  candlestick.    The  ether,  when  lit 
with  a  match  burns  with  an  almost  colorless  flame. 

227.  The  chilling  of  the  flame  can  also  be  brought 

1  Riidorff-Iyiipke,  1.  c.  p.   372. 


172         DEMONSTRATIONS   IN    PHYSICAL   CHEMISTRY 

about  by  diluting  the  gas  with  an  inert  gas  like  nitrogen 
or  carbon  dioxide.  Thus  by  introducing,  through  a  T- 
tube,  dry  carbon  dioxide  in  coal  gas,  the  luminous  flame 
of  a  Bunsen  burner  becomes  colorless. 

228.  The  difference  in  luminosity  of  phosphorus  and 
sulphur,  burning  in  oxygen  and  in  air  also  demonstrates 
strikingly  the  effect  of  temperature  change. 

229.  Diminished   pressure   reduces   the  luminosity   of 
flames.     Thus  the  light  of  a  candle,  burning  in  the  re- 
ceiver of  an  air  pump,  will  become  almost  invisible  on 
quickly  reducing  the  air  pressure.     The  carbon  dioxide, 
formed  in  the  combustion  is  conveniently  removed  by 
placing  a  dish  with  quicklime  in  the  receiver. 

230.  Detonating  gas  ("Knall-gas")  when  exploded  in 
an  eudiometer  or  in  an  explosion  pipette  forms  water 
with  a  luminous  flash,  owing  to  the  enormously  increased 
pressure,  while  in  case  the  gas  mixture  is  bubbled  through 
a  soap  solution  and  a  taper  held  near  the  froth  hardly 
any  flash  is  visible. 

VH.   Explosion  and  Its  Prevention. 

The  question  of  explosion  and  the  means  of  preventing 
gas  mixtures  from  exploding  may  be  treated  in  connec- 
tion with  the  familiar  phenomenon  of  the  "striking  back" 
of  the  Bunsen  flame.  The  principle  on  which  this  burner, 
invented  by  Bunsen  in  1855,  is  built,  is  as  follows:  The 
illuminating  gas,  escaping  from  a  narrow  jet  in  the  base 
of  the  burner,  creates  a  partial  vacuum  around  the  jet 
and  consequently  air  is  drawn  through  the  air  holes  in  the 


COMBUSTION    AND   EXPLOSION 


173 


burner  tube.  By  turning  the  air  regulator,  fitted  with 
two  opposite  holes,  corresponding  to  the  draught  holes  in 
the  tube,  the  air  supply  can  be  varied  at  will. 

231.  The  reduction  of  the  pressure  can  be  made  visible 
by  closing  one  air-hole  and  connecting  the  other  with  a 
small  manometer  (Fig.  76),  containing  an  indigo-solu- 
tion. As  soon  as  the  gas  is  turned  on,  the  liquid  in  the 
gauge  tube  moves  towards  the  burner,  and  moves  back 


Fig.  76. 

again  on  turning  the  gas  off.    If  necessary,  the  movement 
of  the  liquid  can  be  projected  on  the  screen. 

232.  On  fully  opening  the  air  holes,  it  will  be  seen, 
that  the  inner  cone,  distinguished  by  its  green  color, 
burns  with  a  loud  noise  and  moves  constantly  up  and 
down.  This  is  due  to  cross-currents  and  incomplete  mix- 
ing of  the  gases,  as  can  be  proved  by  lengthening  the  tube 
with  another  piece  of  copper  tubing,  20-30  centimeters 
long,  after  which  the  inner  cone  burns  quietly  and  be- 
comes stationary.  It  represents  a  state  of  dynamic  equi- 
librium, which  is  only  disturbed  by  decreased  gas  supply. 


174         DEMONSTRATIONS    IN    PHYSICAL   CHEMISTRY 

On  gradually  turning  off  the  gas,  the  flame  first  begins  to 
flicker  and  finally  strikes  back. 

233.  More  striking  and  directly  visible  to  a  large  aud- 
ience, is  the  following  modification  of  the  preceding  ex- 
periment i1 

A  glass  tube,  4  to  6  feet  long  and  i  to  1^2 
inches  wide,  is  clamped  vertically  over  a  Bunsen  or  Teclu 
burner  and  the  space  between  burner  and  tube  closed  by 
a  plug  of  cotton  wool.  The  top  of  the  tube  is  fitted  with 
a  piece  of  platinum  or  nickel  foil  to  prevent  the  glass 
from  cracking.  The  gas  is  lighted  as  it  issues  from  the 
top  and  burns  with  a  luminous  flame  when  the  air  suppl} 
at  the  base  of  the  burner  is  shut  off.  The  cotton  plug 
is  then  gradually  removed  and  the  blue  Bunsen  flame  is 
obtained.  On  admitting  more  air  by  opening  the  air 
holes  gradually,  or  by  reducing  the  gas  supply  the  blue 
cone  will  recede  down  the  tube.  By  careful  adjustment 
it  can  be  made  to  travel  up  and  down  slowly  at  will. 
From  this  it  may  be  concluded  that  the  flame  of  the 
Bunsen  burner  represents  a  "stationary"  explosion,  in 
which  the  speed  of  the  combustion  wave  is  just  held  in 
equilibrium  by  the  speed  of  the  gas  mixture  moving  up- 
ward. 

234.  In  order  to  show  the  propagation  of  the  combus- 
tion wave,  L,e  Chatelier2  passes  a  mixture  of  nitric  oxide 
and  carbon  bisulphide,    [made  by  allowing  nitric  oxide 
(from  nitric  acid,  specific  gravity  1.2  and  copper  turn- 
ings and  dried  by  concentrated  sulphuric  acid)  to  move 

1  Mellor,  1.  c.  p.  758;  Newth,  1.  c.  p.  230. 

2  1.  c.  p.  273. 


COMBUSTION   AND   EXPLOSION 


175 


over  the  surface  of  a  layer  of  carbon  disulphide,  con- 
tained in  a  small  flask]  through  a  glass  tube,  3-4  meters 
long  and  3  centimeters  wide,  slightly  inclined  under  an 
angle  of  5-10°,  until  the  tube  becomes  colorless  again, 
thereby  indicating  that  the  nitrogen  peroxide,  first  formed, 
has  been  completely  expelled.  The  gas  generator  is  then 
put  aside  and  the  mixture  lit  at  the  upper  end  of  the 
tube.  The  route  of  the  dazzling  white  flame  can  be  fol- 
lowed for  about  1-2  meters,  until  a  sudden  report  indi- 
cates the  transition  of  the  combustion  wave  into  the  ex- 
plosion wave.  The  latter  travels  at  a  speed  of  several 
(two  or  more)  kilometers  per  second. 

235.  H.  Erdmann1  uses  a 
three-necked  Woulfe-bottle 
of  1-1.5  liters  contents,  to 
show  that  not  every  gas 
mixture  is  explosive.  The 
middle  neck  is  provided 
with  a  one-hole  rubber  or 
cork  stopper,  through 
which  passes  a  glass  tube, 
about  i  meter  long  and 
10-15  millimeters  wide. 
Through  the  "~~j  shaped  de- 
livery tube  on  the  left  (Fig. 
77),  illuminating  gas  is 
passed  into  the  bottle,  until 
all  air  has  been  removed, 
the  third  neck  being  closed 
by  a  stopper.  The  gas  is  lit  at  the  mouth  of  the  outlet 


gas 


Fig.  77. 


1  I^ehrbuch  der  anorg.  Chemie,  5e  Aufl.,  p.  433,  (i910)- 


176         DEMONSTRATIONS   IN    PHYSICAL   CHEMISTRY 

tube  and  a  luminous  flame  results.  On  removing  the 
stopper,  the  flame  becomes  non-luminous,  blue,  and  al- 
though the  gas  is  now  mixed  with  air,  there  is  no  ex- 
plosion. On  gradually  turning  off  the  gas  supply  the  blue 
flame  cone  flattens  and  recedes  with  a  concave  surface 
first  slowly,  then  with  increased  speed,  until  after  a  few 
seconds  a  loud  report  is  heard,  resulting  from  the  ex- 
plosion of  the  gas-air  mixture  in  the  bottle.  A  possible 
collapse  of  the  bottle  may  be  counteracted  by  wrapping 
the  bottle  in  strong  copper  wire  gauze,  although  this  may 
seem  to  many  chemists  an  unnecessary  precautionary 
measure. 

236.  An  analogous  experiment  is  the  following,1  per- 
formed with  a  glass  tube,  60  centimeters  long  and  3  centi- 
meters wide,  held  in  a  clamp  at  an  angle  of  about  30°. 
The  upper  end  is  fitted  with  a  one-hole  cork  stopper, 
through  which  passes  a  copper  tube  6  millimeters  in  diam- 
eter and  30  centimeters  long;  the  lower  end  is  closed 
by  a  perforated  stopper,  carrying  the  delivery  tube,  con- 
nected with  the  gas  supply.  The  gas  is  turned  on  and 
lighted  at  the  orifice  of  the  copper  tube.  On  cutting  off 
the  gas  and  removing  the  stopper  at  the  lower  end,  air 
enters  the  tube,  forming  an  explosive  mixture;  the  flame 
retreats  down  the  copper  tube  and  explodes  the  mixture 
in  the  glass  tube.  The  experiment  is  then  repeated,  re- 
placing the  copper  tube  by  another  copper  tube,  with  a  3 
millimeter  bore.  If  properly  adjusted,  the  flame  is  ex- 
tinguished, before  reaching  the  explosive  mixture  in  the 
glass  tube,  which  is  then  lit  at  the  lower  end  by  applying 
a  match. 

1  Mellor,  1.  c.  p.  743. 


COMBUSTION    AND   EXPLOSION  177 

237.  The  following  lecture  experiment,  due  to  Dixon1 
illustrates  the  contrast  between  the  quiet  burning  of  car- 
bon monoxide  and  oxygen  in  a  short  tube,  where  no  ex- 
plosion wave  can  be  set  up  and  the  violent  explosion 
which  takes  place  when  a  wave  is  formed.    First  a  thin- 
walled  test-tube  rilled  with  the  gaseous  mixture  is  ignited 
by  a  taper.    The  quiet  passage  down  the  tube  of  the  blue 
flame  of  the  burning  mixture  can  be  followed  by  the  eye. 
The  tube  is  then  refilled  and  fastened  to  the  end  of  a 
piece  of  lead  tubing  a  few  feet  long,  rilled  with  the  mix- 
ture.   The  test-tube  is  inclosed  in  copper  gauze  and  sur- 
rounded by  a  thick  glass  cylinder.    When  a  flame  is  ap- 
plied at  the  open  end  of  the  pipe,  a  loud  report  is  heard 
and  the  test-tube  is  shattered  to  pieces. 

238.  The  cooling  effect  of  metallic  surfaces  in  keep- 
ing the  temperature  below   the  ignition  point,  may  be 
further  illustrated  by  suddenly  depressing  a  piece  of  cop- 
per wire  gauze  on  a  Bunsen  flame.     The  flame  remains 
for  a  while  entirely  below  the  gauze.     As  the  latter  be- 
comes heated,  the  gas  above  the  wire  catches  fire  after 
a  few  moments. 

238.  The  Davy  safety  lamp,  the  best  known  practical 
application  of  the  metal  wire  gauze  as  a  means  of  pre- 
venting explosion,  may  be  demonstrated  and  its  useful- 
ness illustrated  by  lowering  it  into  a  highly  explosive 
mixture  of  ethyl  ether  and  air,  contained  in  a  2-liter 
beaker.  The  flame  inside  the  lamp  is  extinguished  after 
a  few  moments,  but  no  explosion  results. 

1  Mellor,  Chemical  Statics  and  Dynamics,  I^ondon,  p.  485,  (1909). 


178         DEMONSTRATIONS    IN    PHYSICAL   CHEMISTRY 

240.  The  same  principle,  slightly  modified,  is  also  ap- 
plied in  the  construction  of  the  burner,  invented  by 
Meker.  On  taking  the  burner  apart,  it  will  be  seen  that 
the  air  holes  (four  or  five,  instead  of  two)  are  unusually 
large,  so  that  really  an  explosive  mixture  of  gas  and  air 
is  formed.  No  striking  back  is  observed,  however,  on 
lighting  the  gas,  since  a  deep  nickel  grid,  closing  the  en- 
larged outlet  of  the  burner  tube,  exerts  its  cooling  effect, 
thereby  preventing  the  flame  from  striking  back. 


CHAPTER  XII. 


LIQUID  AIR  EXPERIMENTS. 

The  experiments  which  can  be  carried  out  with  the  aid 
of  liquid  air  are  among  the  most  striking  that  can  be 
performed  in  chemistry  courses.  Since 
liquid  air  is  on  the  market  nowadays 
at    reasonable    prices,1    there    is    no 
obstacle  in  the  way  of  performing  a 
number  of  interesting  low-temperature 
experiments  which  can  all  be  shown 
in  a  i-  or  2-hour  period. 

Liquid  air  can  be  stored  for  quite  a 
while  when  kept  in  the  double-walled 
vessels  with  an  evacuated  space  be- 
tween, first  introduced  by  Dewar.2  Re- 
cently Weinhold  has  devised  vessels 
with  four  walls,  having  three  air-free 
layers  between  the  liquid  air  and  the 
outside  atmosphere  (Fig.  78).  A 
further  improvement  for  the  conser- 
vation of  liquid  air  has  been  made  by 
silvering  the  inner  walls  in  order  to 
keep  off  radiant  heat.  In  this  way, 
it  has  been  made  possible  to  store 
quantities  of  1-2  liters  of  liquid  air  for 
about  8-14  days,  the  daily  loss  being  about  100  cc.  Liquid 
air  should  never  be  poured  from  one  vessel  into  another 

1  Not  long  ago  (March,  1915)  the  firm  of  A.  R.  Ahrendt  &  Co.,  in  Berlin, 
offered  liquid  air  for  sale  at  the  price  of  50-60  f  per  liter. 

2  Proc.  Royal  Inst.  14,  p.  i,  (1893). 


Fig.  78. 


l8o         DEMONSTRATIONS    IN    PHYSICAL,   CHEMISTRY 

since  it  happens  very  often  that  the  vessel  from  which 
the  air  is  taken  cracks  at  the  junction  of  both  walls. 
For  this  reason  it  is  necessary  to  transfer  the  liquid  air 
by  blowing  it  from  one  vessel  into  another  by  means  of 
a  rubber  balloon,  as  shown  in  Fig.  79.  Care  should  be 


Fig.  79. 

taken,  that  the  two-hole  cork  stopper,  carrying  the  glass 
tubes,  is  not  pressed  too  tightly  into  the  neck  of  the  bal- 
loon or  jar,  containing  the  air. 

The  experiments  to  be  performed  are  conveniently  di- 
vided in  two  sets :  First — those  illustrating  purely  phys- 
ical properties  of  matter  at  very  low  temperature  (liquid 


LIQUID  AIR  EXPERIMENTS  181 

air  boils  at  about  — 190°)  and  in  the  second  place — those 
illustrating  chemical  reactions  at  extreme  temperatures, 
depending  on  the  fact  that  liquid  air  is  a  source  of  oxygen. 
Liquid  air  when  freshly  prepared  contains  about  30  per 
cent,  of  oxygen,  but  on  standing  it  gradually  becomes 
richer  in  oxygen  (up  to  55  per  cent.)  due  to  the  fact,  that 
nitrogen  having  a  lower  boiling  point  ( — 195°)  evap- 
orates more  rapidly  than  oxygen  (boiling  point  — 182°). 

I.  Physical  properties  of  matter  at — 190°. 

241.  Most  of  the  familiar  gases  change  into  almost 
colorless  solids  when  cooled  to 
the  temperature  of  liquid  air. 
Two  500  cc.  glass  balloons,  with 
hollow  bottoms,  (Fig.  80),  con- 
taining dry  chlorine  and  dry 
bromine  gas  respectively,  are 
sealed  and  then  held  upside  down 
by  clamps  between  two  thick- 
walled  glass  plates  placed  par- 
allel to  each  other,  so  that  in  the 
event  of  an  explosion  the  lectur- 
er and  his  auditory  are  sufficient- 
ly protected.  Liquid  air  is  cau- 
tiously poured  in  the  cavities  of 
the  bottoms,  with  the  result  that 
both  gases  instantly  solidify 
(freezing  point  of  chlorine 

—102°,  of  bromine  — 7°).  An  almost  perfect  vacuum 
is  created,  no  gas  being  left  in  the  balloons  which  appear 
absolutely  colorless. 


l82         DEMONSTRATIONS   IN    PHYSICAL   CHEMISTRY 


Z42.  That  a  high  vacuum  can  be  effected  in  this  way 
is  clearly  shown  by  solidifying  carefully  dried  carbon  di- 
oxide gas  in  a  T-shaped  tube  (length  of  horizontal  limb 
20  centimeters,  of  vertical  limb  22  centimeters,  diameter 
3  centimeters).  The  tube  is  provided  with  two  platinum 


Fig.  81. 

electrodes  to  which  are  soldered  two  short  aluminium 
wires  2  centimeters  long  and  3  millimeters  thick,  (Fig. 
81).  The  electrodes  are  connected  with  the  secondary 
poles  of  an  inductorium.  The  distance  between  the  alu- 
minium wires  (about  15  centimeters)  is  too  large  to  allow 


UQUID   AIR   EXPERIMENTS 


the  passage  of  sparks,  but  on  slowly  lowering  the  vertical 
limb  of  the  tube  into  a  Dewar  vessel,  half  filled  with 
liquid  air  (a  preliminary  cooling  is  effected  by  the  escap- 
ing air  vapor,  before  the  tube  is  lowered  in  the  liquid 
air)  the  gas  immediately  changes  into  a  white  snow,  de- 
posited against  the  walls  of  the  vertical  limb.  At  the 
same  time  a  brilliant  yellowish  green  spark  light  be- 
comes visible,  increasing  in  intensity  the  more  the  tube 
cools.  Darkening  of  the  lecture  room  helps  to  demon- 
strate this  striking  phenomenon  in  all  its  splendor. 

243.  The  freezing  of  water  by  its  own  evaporation  is 

A 


Fig.  82. 

brought  about  instantly  in  liquid  air.  Two  glass  bulbs, 
about  4  centimeters  in  diameter  are  connected  by  a  glass 
tube,  30  centimeters  long  and  0.8  centimeter  wide,  twice 
bent  at  right  angles  (Fig.  82).  One  of  them  is  partly 
filled  with  water.  The  air  is  completely  driven  out,  so 
that  the  whole  apparatus  contains  nothing  but  water  and 
water  vapor.  On  plunging  the  vapor-bulb  in  liquid  air, 
13 


184         DEMONSTRATIONS   IN    PHYSICAL   CHEMISTRY 

the  water  in  the  second  bulb  freezes  at  once.  It  may  be 
remarked  that  this  apparatus,  Wollaston's  "cryophorus,"1 
gives  good  results  even  at  much  higher  temperatures 
( — 20°)  although  not  as  readily  as  in  liquid  air. 

244.  The  freezing  of  liquids,  like  ether  and  alcohol, 
may  be  shown  next.    An  alcoholic  solution  of  iodine  is 
transformed  into  a  viscid  glass-like  material  which  con- 
tracts on  solidifying.    At  the  same  time  it  becomes  yel- 
lowish-orange in  color  while  in  the  liquid  state  the  solu- 
tion is  distinctly  brown. 

245.  Many  solid  substances  change  color  when  cooled 
in  liquid  air,  proving  that  the  power  of  absorbing  light 
is  considerably  modified  at  low  temperature.2    Crystals  of 
sulphur,  potassium  bichromate  and  cinnabar  may  be  used 
as  examples. 

246.  Rubber,  when  brought  to  the  temperature  of  liq- 
uid air  becomes  as  brittle  as  glass  and  may  be  ground  in 
a  mortar.    The  same  is  true  for  grapes,  meat,  lard,  etc. 
On  allowing  the  temperature  to  rise,  all  these  substances 
regain  their  original  properties. 

247.  Metals  become  stronger  towards  a  steady  pull.    A 
coil  of  lead  wire,  3  millimeters  in  diameter,  when  cooled 
in  liquid  air  is  able  to  hold  a  weight  of  300  grams  with- 
out considerable  stretching.    Rise  of  temperature  causes 
the  wire  to  stretch  and  finally  to  become  almost  straight 
on  returning  to  room  temperature. 

i  Graham,  Elements  of  Chemistry,  ed.  Watts  and  Bridges,  p.  75,  (1866.) 
*  Kreuz,  Phil.  Mag.,  (5)  39,   p.  209,  (1895). 


LIQUID  AIR  EXPERIMENTS  185 

248.  On  striking  a  small  lead  disk  (or  bell)  cooled  to 
—i 80°  with  a  wood  hammer  a  clear  metallic  sound  is 

heard. 

249.  Liquid   air,  when  kept   for   several   hours   in  a 
Dewar  vessel  is  heavier  than  water.    This  is  due  to  the 
fact  that  it  contains  then  about  40-50  per  cent,  oxygen, 
with  a  specific  gravity  of  1,135  (at  — 182°)  against  60- 
50  per  cent,  nitrogen  with  a  specific  gravity  of  0.885  (at 
—195°).    On  pouring  20-30  cc.  of  liquid  air  in  a  beaker 

of  water,  a  few  drops  of  air  are  seen  floating  on  the 
water,  separated  by  a  layer  of  gaseous  air  (phenomenon 
of  L,eidenfrost),  but  occasionally  some  drops  sink  down 
and  rise  again  by  the  continuous  formation  of  gas. 

250.  The  usefulness  of  liquid  air  for  the  creation  of 
high  vacua  is  due  to  the  fact,  that  the  power  of  char- 
coal to  absorb  gases  is  greatly  increased  at  low  tempera- 
tures.   Thus  Dewar  found,1  that  i  cc.  of  charcoal  at  o° 
takes  up  4  cc.  of  hydrogen  or  18  cc.  of  oxygen,  at  — 185° 
it  takes  up  135  cc.  of  hydrogen  or  239  cc.  of  oxygen.  The 
charcoal  obtained  from  the  shells  of  the  cocoa-nut  is  es- 
pecially adapted  for  this  purpose.    A  tube,  filled  with  this 
charcoal   (previously  heated  to  drive  off  absorbed  gases 
and  moisture),  is  sealed  to  another  tube  (dimensions  20 
by  3.5  centimeters)  containing  dry  nitrogen,  and  provided 
at  both  ends  with  platinum  wires  to  which  are  soldered 
short  aluminium  electrodes.     When  connection  is  made 
with  the  secondary  poles  of  an  inductorium   (Fig.  83), 
nothing  except  an  occasional  flash  is  observed,  showing 

i  Proc.  Royal  Soc.,  74,  p.  126  (1904)  ;  Chem.  News,  94,  p.  173  (1906). 
14 


l86         DEMONSTRATIONS   IN    PHYSICAL   CHEMISTRY 

that  absorption  of  nitrogen  has  not  taken  place  to  any 
marked  extent.  On  slowly  lowering  the  limb  containing 
the  charcoal  into  a  Dewar  vessel  filled  with  liquid  air, 
forked  brush-like  reddish  sparks  begin  to  leap  from  elec- 
trode to  electrode.  As  the  gas  absorption  proceeds  there 
appears  a  luminous  band  breaking  up  (at  a  pressure  of  3 
millimeters)  into  a  number  of  striae,  while  at  the  same 
time  a  dark  space,  Faraday's  dark  space,  forms  around  the 


Fig.  83. 

cathode.  Finally  after  about  10  minutes,  when  the  press- 
ure has  sunk  to  about  0.03  millimeter  of  mercury  the 
striae  disappear,  and  a  dark  space,  starting  from  the  ca- 
thode, "Crookes'  dark  space,"  expands  and  fills  the  tube. 
A  green  fluorescent  light  is  emitted  from  the  walls  around 
the  cathode  which,  on  reverting  the  primary  current, 
switches  over  to  the  other  end  of  the  tube. 


LIQUID   AIR   EXPERIMENTS  187 

II.  Chemical  properties  of  matter  at  — 190°. 

251.  Chemical  action  at  — 190°  is  hardly  perceptible; 
the    molecules     are,     according    to    Dewar,    near    to 
the  "death  of  matter."    When  a  small  piece  of  sodium 
and  a  few  cubic  centimeters  of  strong  hydrochloric  acid 
are  cooled  separately  in  two  test-tubes  in  liquid  air  and 
then  brought  in  contact  with  each  other,  no  reaction  takes 
place.1 

252.  Liquid  air,  on  standing,  becomes  rich  in  oxygen. 
This  accounts  for  the  fact  that  a  burning  taper,  when 
plunged  into  some  liquid  air,  contained  in  a  beaker,  is  not 
extinguished,  but  burns  vigorously  notwithstanding  the 
extremely  low  temperature  in  the  beaker.    In  performing 
this  experiment,  proper  precautions  must  be  taken,  the 
beaker  in  most  cases  being  reduced  to  pieces. 

253.  A  plug  of  cotton  wool  is  treated  with  powdered 
charcoal  so  that  the  latter  becomes  finely  divided  in  the 
cotton.     The  preparation  held  in  the  loop  of  platinum 
wire,  is  then  dipped  into  liquid  air  for  a  few  minutes. 
On  igniting  the  cotton-charcoal' in  a  flame  the  mixture 
burns  like  gun  cotton,2  with  an  intensely  bright  flame. 

1  Pictet,  Comptes  rendus  115,  p.  814  (1892). 
8  Erdmanii,  1.  c.  p.  243. 


BIBLIOGRAPHY. 


The  following  is  a  list  of  the  literature  references,  frequently 
quoted  in  the  text: 

Emil  Baur,  Themen  der  physikalischen  Chemie,  Leipzig  (1910). 
S.  Lawrence  Bigelow,  Theoretical  and  Physical  Chemistry,  New 
York  (1914). 

H.  und  W.  Biltz,  t)bungsbeispiele  aus  der  unorganischen  Experi- 

mental-Chemie,  Hamburg  (1907). 
Alfred    Coehn,    Electro-Chemie,   in   Miiller-Pouillet's   Handbuch 

der  Physik,  IVer  End.,  Leipzig  (1909). 
H.    Erdmann,    Lehrbuch    der    anorganischen    Chemie,    5e    Aufl., 

Braunschweig  (1910). 

E.  Hatschek,  Physics  and  Chemistry  of  Colloids,  London  (1913). 
Karl  Heumann,  Anleitung  zum  Experimentieren,  36  Aufl.,  von 

O.  Kuhling,  Braunschweig  (1904). 

Louis  Kahlenberg,  Outlines  of  Chemistry,  revised  ed.,  New  York 
(1916). 

F.  W.  Kiister,  Zeitschr.  f.  Electrochemie,  4,  pp.  105-113  (1898). 
H.  Le  Chatelier,  Lemons  sur  le  carbone,  Paris  (1908). 

R.  Liipke,  Grundziige  der  Electro-Chemie,  5e  Aufl.,  von  E.  Bose, 

Leipzig  (1907). 
R.  Luther,  Die  chemischen  Vorgange  in  der  Photographic,  Halle 

(1899). 

R.  Meldola,  The  Chemistry  of  Photography,  London  (1891). 
J.    W.    Mellor,    Modern    Inorganic    Chemistry,    new   impression, 

London  (1016). 

G.  S.  Newth,  Chemical  Lecture  Experiments,  revised  ed..  London 


A.  A.  Noyes,  Journ.  Am.  Chem.  Soc.,  27,  pp.  85-104  (1905). 
A.  A.  Noyes  and  A.  A.  Blanchard,  Ibidem,  22,  pp.  726-752  (1900). 
A.  A.  Noyes  and  G.  V.  Sammet,  Ibidem,  24,  pp.  498-515  (1902). 


BIBLIOGRAPHY  189 

Wilhelm    Ostwald,    Grundlinien   der   anorganischen    Chemie,   36 

Aufl.,  Leipzig  (1912). 
Wilhelm  Ostwald,  Grundrisz  der  allgemeinen  Chemie,  46  Aufl., 

Leipzig  (1909). 
Wilhelm  Ostwald,  Die  wissenschaftlichen  Grundlagen  der  analy- 

tischen    Chemie,    5e   Aufl.,    Leipzig    (1910).     Translated   by 

G.    McGowan :     The    Scientific    Foundations    of    Analytical 

Chemistry,  3rd  ed.,  London  (1908). 
Wolfgang   Ostwald,    Handbook   of    Colloid-Chemistry;    English 

translation  by  Martin  H.  Fischer,  Philadelphia  (1915). 
E.    Riidorff,    Grundrisz   der   Chemie,    I2e   Aufl.   von    R.    Lupke, 

Leipzig  (1902). 
Alexander  Smith,  Introduction  to  Inorganic  Chemistry,  3rd  ed., 

New  York  (1917). 
Julius  Stieglitz,  Elements  of  Qualitative  Chemical  Analysis,  vol. 

I,  New  York  (1916). 


INDEX  TO  AUTHORS.* 


Arrhenius,  67. 

Bakhuis  Roozeboom,  2,  54. 

Bancroft,  2,  54,  170. 

Baur,  44,  63. 

Benrath,  149. 

Bergmann,  159. 

Berthollet,  149. 

Berzelius,  55. 

Bigelow,  S.  L.,  125. 

Biltz,  H,  16. 

Biltz,  W.,  125. 

Bingham,  13. 

Blanchard,  50,  52,  75,  76,  93, 

95,  H2. 
Bone,  159. 
Brann,  43. 

Bredig,  61,  65,  116,  117. 
Bucholz,  89. 
Bunsen,  149,  172. 

Cady,  106. 
Ciamician,  36,  149. 
Coehn,  130,  150,  151. 
Cohen,  E.,  8. 
Crafts,  56. 
Crookes,  186. 
Crum  Brown,  28,  98. 

Daniell,  88. 

Davy,  159. 

Deville,  41,  170. 

Dewar,  140,  179,  185,  187. 

*  Text  references  only. 


Dixon,  177. 
Dolezalek,  89. 
Donnan,  127. 
Draper,  149,  151. 
Duclaux,  125. 
Dulong,  3,  12. 

Eder,  150. 
Erdmann,  H.,  175. 
Eykman,  34,  36. 

Faraday,  n,  12,  118,  186. 
Fenton,  6. 
Fichter,  134. 
Fischer,  E.,  40. 
Frankland,  159. 
Freer,  20. 
Fremy,  144. 
Freundlich,  133,  136. 
Friedel,  56. 

Gattermann,  8. 
Gay-Lussac,  54. 
Gemberling,  125. 
Gibbs,  J.  W.,  2,  128. 
Gladstone,  42. 
Goppelsroeder,  134. 
Gouy,  151. 
Graham,  15,  18,  20,  21,  114, 

120,  124. 
Gutbier,  118. 

Haber,  159,  162,  165. 


INDEX    TO    AUTHORS 


191 


Handa,  51. 
Henry,  V.,  121,  125. 
Heumann,  159. 
Heydweiller,  100,  102. 
Heyer,  124. 
Hofmann,  A.  W.t  67. 
Huston,  146. 

Ingle,  164. 

Kahlenberg,  29,  105. 
Keir,  II. 
Kenrick,  42. 
Kohlrausch,  100,  102. 
Kruger,  89. 
Kruyt,  10. 
Ktister,  73,  78,  82. 

Landolt,  48,  49. 
Landsberger,  34. 
Lash  Miller,  42. 
Le  Chatelier,  174. 
Leidenfrost,  185. 
Lermontoff,  82. 
Leveing,  44. 
Lewes,  159. 
Lhermite,  29. 
Lichtenwalter,  106. 
Lodge,  70,  74. 
Liipke,  25,  78,  80,  84,  85, 

171. 
Luther,  44,  65. 

Malfitano,  124,  125. 
Marino,  156. 
McCoy,  34. 
Meker,  178. 


Mellor,  54. 
Meyer,  V.,  35. 
Morse,  H.  N.,  26. 
Miiller,  145. 

Nernst,  27,  51,  54,  68,  71,  73, 

74,  112,  113. 
Newth,  109,  156,  168. 
Nollet,  25. 
Noyes,  A.  A.,  43,  50,  52,  55, 

56,  57,  60,  75,  76,  93,  95, 

112,   113,  122,   131,  136. 

Ober,  144. 

Ostwald,    Wilhelm,    18,    54, 

55,  97,  99,  107- 

Ostwald,  Wolfgang,  10,  114, 
115,  122. 

Palmaer,  69. 
Pebal,  6. 
Perrin,  121. 
Petit,  3,  12. 
Pfeffer,  26. 
Plotnikov,  149. 
Porlezza,  156. 
Priestley,  159. 
Priwoznick,  153. 

Quartaroli,  62. 

Ramsay,  24. 
Reusz,  133. 
Richardt,  162. 
Rigollot,  151. 
Roberts  Austen,  22. 
Roscoe,  149. 


192  INDEX   TO   AUTHORS 

Sahlbohm,  134.  Treadwell,  100. 

vSammet,  55,  56,  57,  60.                     Tyndall,  129. 
Scheele,  159. 

Schonbein,  n.  Van  Bemmelen,  144. 

Schoorl,  103.  Van  't  Hoff,  2,  38,  44. 

Scriba,  92.  Voigtlander,  123. 

Silber,  149.  Volta,  91. 

Smithells,  159,  164,  165.  Von  Dieterich,  45. 
Smith,  A.,  10,  54. 

Smits,  12.  Wassiljewa,  150. 

Soret,  165.  Weinhold,  179. 

Spitalski,  65.  Weiser,  170. 

Stieglitz,  47,  93.  Whitney,  95,  144. 

Svedberg,  116.  Wilke,  65. 

Wohler,  F.,  16. 

Tammann,  2,  3,  26.  Wohler,  L,.,  45. 

Teclu,  164.  Wollaston,  184. 
Than,  6. 

Tolloczko,  no.  Zsigmondy,  118,  124,  147. 
Traube,  M.,  25. 


INDEX  OF  SUBJECTS, 


Actinometers,  150,  151. 

Adsorption,  of  gases,  140;  — of  liquids,  141;  — of  dissolved  sub- 
stances, 141-143;  selective — ,  143-146;  mutual —  of  colloids, 
146-148  . 

AUotropy,  of  tin,  8 ;  dynamical —  of  sulphur,  10. 

Atmolysis,  6,  18. 

Atomic  heat,  of  tin,  13;  — of  lead,  13. 

Autocatalysis,  61-63. 

Blue  print  process,  153. 

Capillary  analysis,  134. 

Carburetion  of  non-luminous  flames,  167,  168. 

Catalysis,  55-66. 

Cataphoresis,  131. 

Chemical  properties  of  matter  at  — 190°,  187. 

Collodion  tubes,  use  of —  for  dialysis,  125. 

Colloid  solutions,  preparation  of — ,  116-122. 

Combination  cells,  83. 

Combustion  of  gases,  160,  161. 

Common  ion  effect,  98-100. 

Concentration  cells,  88,  89. 

Conductivity,  of  water,  91 ;  — of  hydrochloric  acid  in  toluene,  92 ; 

— in  water,  93,  94;  — of  sodium  chloride,  92. 
Crystallization  luminescence,  of  arsenious  acid,  155;  — of  sodium 

fluoride,  155. 

Davy's  safety  lamp,  177. 

Degree  of  dissociation,  94-97. 

Depolarizers,  88. 

Devitrification,  of  sodium  silicate  glass,  7. 

Dialysis,  123-125. 


194  INDEX   OF   SUBJECTS 

Diffusion,  of  bromine,  16;  —of  carbon  dioxide,  16;  — of  copper 
nitrate,  21;  —of  hydrogen,  15;  —in  gelatine  plates,  22;  speed 
of — ,  122,  123. 

Disintegration  of  noble  metals  in  water,  117,  118. 

Dispersoids,  mechanical  properties  of—,  122-128;  optical  proper- 
ties of—,  128,  129;  electrical  properties  of—,  129-139. 

Displacement  cells,  78-83. 

Dissociation,  of  ammonium  chloride,  5. 

Draper's  law,  151. 

Dulong  and  Petit's  law,  12. 

Effusion,  of  hydrogen,  20;  — of  oxygen,  20. 

Electrolysis,  of  stannous  chloride,  68;  —of  potassium  chloride 

68. 

Electromotive  chemistry,  78-91. 
Electrophoresis,  131. 
Emulsoids,  preparation  of — ,  121. 
Endosmosis,  133. 
Equilibrium  displacement,  46,  47. 
Explosion  and  its  prevention,  172-178. 

Fever  reactions,  63. 

Flames,    structure    of—,    162,    163;    chemical    composition    of—, 

164-166. 
Fluorescence,  of  fluorescein,  155;  — of  eosin,  155;  —of  quinine 

sulphate  solutions,  156. 

Galvanic  liquid  cells,  89,  90. 
Hydrolysis,  100-104,  120. 

lonization,  and  chemical  activity,  104-106;  — and  color  of  solu- 
tions, 106,  107. 

Liquid  crystals,  8. 

Liquid  air  experiments,  179-187. 


INDEX   OF   SUBJECTS  IQ5 

Luminosity  of  flames,  in  the  presence  of  solid  particles,  166-168; 
— without  solid  particles,  170,  171;  changes  in — ,  171,  172. 

Mass  action  law,  42-45. 

Migration  of  ions,  69-78. 

Molecular   weight   determination,   by   the   ebullioscopic   method, 

34;  — by  the  vapor  density  method,  35;  — by  the  cryoscopic 

method,  36,  37. 
Mutual  precipitation  of  colloid  solutions,  135. 

Negative  catalyzers,  63. 

Osmotic   experiments,   with   ammonia-air   mixtures,   24;    — with 

sugar  solutions,  25,  27;  — with  three  liquids,  26-29. 
Oxidation  cells,  83. 

Passivity,  of  iron,  12. 

Peptization,  120. 

Phase  transitions,  2. 

Phosphorescence,  of  phosphorus,  156-158. 

Photochemical  decomposition,  of  hydrogen  chloride,  150;  — of 
silver  chloride,  152;  — of  cuprous  chloride,  153. 

Photochemical  extinction,  150,  151. 

Photochemical  synthesis  of  hydrogen  chloride,  150. 

Physical  properties  of  matter  at  — 190°,  181-186. 

Pigment  process,  153. 

Plasticity  of  clay,  I. 

Polarization,  84-86. 

Polymorphic  transformation,  of  cuprous  mercuric  iodide,  3 ;  —of 
mercuric  iodide,  4 ;  — of  potassium  bichromate,  5 ;  — of  potas- 
sium tungstate,  4. 

Precipitation  membranes,  of  various  silicates,  26. 

Protective  colloids,  138. 

Reduction  methods  for  preparing  hydrosols,  118. 
Reversible  reactions,  39-41. 


196  INDEX    OF   SUBJECTS 

Semipermeable  membrane  of  copper  ferrocyanide,  25. 

Separation  of  solids  from  flames,  168-170. 

Solubility,  of  liquids,  108,  109;  —of  one  solute  in  two  solvents, 

109-111;  — of  salts  in  water,  111-113. 
Soret's  optical  test,  165. 
Strength  of  acids,  98. 
Successive  reactions,  54. 
Supercooling,  6. 

Surface  tension,  of  emulsoids,  126-128. 

Suspensions,  preparation  of — ,  121 ;  deflocculation  of — ,  138. 
Suspensoids,  preparation  of — ,  116-121. 

Time  reactions,  48-49. 

Tribe-luminescence,  of  uranium  nitrate,  155;  — of  salophen,  155. 

Tyndall  phenomenon,  129. 

Undercooling,  of  sodium  thiosulphate,  6. 

Vapor  pressure,  of  water,  alcohol  and  ether,  31 ;  — of  solutions 
of  benzoic  acid  in  ether,  32;  lowering  of —  by  dissolved  sub- 
stances, 31-33- 

Velocity  of  chemical  reactions,  50-53. 

Viscous  character,  of  pitch,  I. 

Viscosity,  of  suspensoids,  125,  126;  — of  emulsoids,  125,  126. 


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